SurvivingInfidelity.com^®

Hello,

I (28 F)am hoping to gain some insight from those who have gone through similar situations. I just recently discovered my sister (31 F) is cheating on her husband (32 M). They have been together for about 6 years now, and she is sleeping with a co-worker. I had a hunch because she was acting strangely around both her husband and this other man.

I approached her about it privately. She did not immediately admit to me about what she was doing. She has been caught texting this guy before, and it caused major problems in the relationship about 6 months ago. Her husband has since become depressed due to their failing relationship and now is on medication for depression and sleeping meds for anxiety. He does not know that my sister continues to talk with this co-worker--much less sleep with him.

I know my sister is not happy in her marriage. But her husband is a great guy and does not deserve to be cheated on. I care for my sister deeply, and do not wish to hurt her. I fear she is hurting herself though. I figure if she does not do something to address this, it is going to come out eventually, and then she will have zero power to control the situation.

For a few days after I found out about her affair, my sister avoided me for fear that I would tell the rest of our family (I live in a different country and rarely visit). I did not tell anyone else because I don't feel that would be right, and could hurt a lot of people. I don't want to ruin my sister's life, but I fear she is doing that on her own. I was able to figure out her having an affair the first day I was back in town. It astounds me that everyone else has been blind to it. But it's just a matter of time before someone finds out. Her husband already suspects because she is so distant towards him. She has mentioned divorce but neither one seem to be serious about it.

I spoke with her on the phone yesterday. I initially told her that she has a few months to figure it out before I say anything to her husband or anyone else. She became very upset and said I was blackmailing her. That I was taking away the only thing in life that made her happy. She told me that the only thing she wants is happiness, and the only person that brings that to her is her co-worker. I did not want to upset her, so I agreed to not give her a timeline or say anything at the moment because I can tell she is hurting. But I also don't think she realizes how much she is hurting other people, including her husband and the wife and children of her co-worker. Additionally, this could affect the family company (for which my mother, brother, sister, and father all have their livelihoods from). I told her that if she is not happy in the relationship, she needs to get a divorce. She said that she is afraid she would have to split custody of her daughter, and is too afraid of change and losing the house to get a divorce. So I told her if that was the case, she needs to stop sleeping with her co-worker and work on her marriage. She is not willing to do this either. She knows that the co-worker might only be sleeping with her because she is his boss. She also knows that this man will not leave his wife and three children for her. The conversation ended by me asking how I can support her, and what she would do if the roles were flipped. She said "just listen and try to understand". I then asked her if she was angry with me. She said yes because she feels as though I am trying to ruin her life.

I want to be supportive of her. I want my sister to be happy. No one deserves to feel miserable. BUT I feel what she is doing is very wrong and will seriously hurt a lot of people if it continues. I feel I may be enabling her by having knowledge of what is going on and not taking action. But I have also seen online where a majority of people have recommended to "mind your own business and don't butt into their relationship". So I seek wisdom from this forum. Both sides please, what should I do?

A few more details: the guy my sister is cheating with is married and has 3 children, the youngest of which is 5 months old. My sister has one 2 year old daughter with her husband. My sister is a part-owner at our family company. The co-worker she is sleeping with handles all the finances of our family's company (yikes). I visit again in 3 weeks. I can easily keep this a secret if need be, but I don't feel this is the right answer. I was hoping she would agree to stop seeing her co-worker and work on her marriage, or get a divorce with her husband. Her husband has never shown any abusive behavior, and she denied that he ever hit or hurt her. They do bicker often, especially since having their daughter 2 years ago.

Welcome to SI, I'm sorry you have had to carry this burden. Your Sister is very selfish, only thinking of herself. This forum is full of people who have been victimized by infidelity. Prior to being betrayed by my Wife I had the same "mind your business" opinion that many people have. Now everyone around me knows, if I catch infidelity I'm exposing it.

She is putting your family at risk, she is doing it for her "happiness". She needs to grow up, happiness doesn't come from destroying people and families. You need to tell her Husband and direct him here to SI. Do not tell your Sister about this place or that you are exposing her affair.

Also, the fallout and damage is 100 on her and coworker, if lives are torn apart its her fault. I wish you the best

I’ve been on this forum since 2013 and have read through, literally, thousands of infidelity posts, and I am also in a profession that deals a lot with people who have various forms of addiction, and I am blown away at how so closely infidelity resembles addiction. Cheaters in the throes of their affair think, behave and "rationalize" very much like heroin, meth or alcohol addicts. They’re self destructive, desperate, selfish, impulsive, mood swings, single minded, preoccupied, neglect responsibilities, neglect relationships, risk taking, cravings, lying, manipulation, blame shifting, and… the inability to stop regardless of the horrific consequences.

And, they use the common language of the addicted:

"I have it under control."
"I can stop anytime I want, nobody has to know"
"You are just jealous because I can have fun and you can’t."
"You just don’t want me to be happy."
"You don’t even try to understand how I feel."

Now, with that said, I hate to put the addiction label on cheating because it allows the cheater to assume a victim role and disassociate from their "uncontrollable" behavior, but damn, they sure do behave like addicts.

She’s destroying two families with young children. Her actions will forever affect her children’s lives and development. She’s deeply traumatizing her husband. She’s endangering her relationship with you and her origin family and she’s risking the family business…FOR WHAT? There’s no future in this. There’s no happy ending. She’s living in the momentary high, chasing her next fix, tuning out from reality, just like an addict.

Family intervention time.

Even if you are torn about your loyalty to your sister. Your mother and fathers livelihood depends on this business. This lying, cheating OM has access to their business finances.

In that situation why wouldn’t you expose the affair? If he damages your family finances how will your parents manage?

I do not envy your situation but I would protect the family business.

Her husband, who I presume you like, has failing mental health due to her actions. This will impact on your niece. Infidelity can cause ptsd. He needs to know so he can protect himself (and also get checked for stds).

Your sister is acting irrationally (like a cheater does) she is not protecting anyone other than herself. I think her poor judgement leaves you little choice. I’d tell her husband and then I’d tell your parents so they can do what they can to ensure the business is safe.

I am sorry you are in this situation. It’s rubbish.

Also happiness.
Your sister may think she deserves happiness. Why does she think her husband doesn’t deserve happiness.

I think cheaters cause their own unhappiness. They rewrite history and look out for their spouses bad points during an affair to justify their poor choices. She will look at the man crumbling in front of her and not think ‘I caused that’. She will use it to justify her poor choices and compare him unfavourably to the work colleague who is sleeping with (at least) two women and having the associated ego boost. He will be bouncy and flattering and giving her ego boosts. While her husband knows something is wrong and is been gaslit and has failing mental health.

If she had focused her attention and time on her husband and child maybe she would be happy. If she isn’t then leave.

I'd like to write my thoughts about your relationship with your sister. Exposing her may make you feel afraid that you will loose your friendship. But would a friend demand to help covering such ugly betrayal? Understand? There is nothing understandable in what she is doing. She's not in the mindset to be anyones good sister, daughter, friend, mother or spouse. She's not a good person right now. If you expose her, she will have to deal with her brokenness and if she is not a narc, she will eventually come to you and apologise.

[This message edited by Potentialforevil at 9:54 AM, Thursday, November 30th]

Blove

From a purely business angle: Your sister is placing the company in the line-of-fire…

Relationships between subordinates and their managers are heavily frowned upon. If your sister is this guys manager or can in any way impact his job (positively or negatively) as the owner (as in a member of the board) then she had committed a managerial faux-pas that is probably the business equivalent of infidelity…

If there is anyone in a company that you want to have 100% integrity then that’s the person managing your money… Heck… I recall one financial manager for a large government organization that went berserk when presented with a cake on his retirement. Like he pointed out – there was no official approved budget for cake. This is the type you want managing cash.

Consider sharing this info with the CEO or the COB. Not for personal reasons, but purely business.

On a personal level: Allowing your sister to hide behind the happiness excuse… would you allow her to shoot up heroin because it gave her 10 minutes of ecstasy? Her form of happiness is an unhealthy and unconstructive form of happiness. It’s at the cost of three other people, not counting her kid and the three other kids.

I get it that some marriages die. That OK per se. Divorce is something that happens. But point out to her that being in an affair while still not ending her marriage is totally unsustainable. She has two options basically:

She ends her marriage.

She ends the affair and reconciles her marriage.

As far as work goes… this will lead to one or both having to leave their jobs.

Not unless both divorce and then commit to each other. But this does create a terrible situation for the company.

Edited to add:
If your company is fortunate enough that the financial manager is hierarchically superior to your sister… fire him.
He won’t have a claim to wrongful dismissal (which he will if roles are reversed), and frankly his moral decisions would make me worry about his ethical ability to manage my cash.

[This message edited by SI Staff at 11:15 AM, Thursday, November 30th]

You need to quietly and consistently act in accordance with your principles.

Don’t get sucked into being an accomplice, providing cover, or lying. Don’t Mollycoddle her. I think you should stick to the approach that you are telling your family and her husband in short course, as well as his family, and you are giving her just enough grace to tell them before you do.

[This message edited by straightup at 1:52 PM, Thursday, November 30th]

Lots of great advice here for you in a difficult situation. I would add that you and hopefully your sister need to realize a key point: very often the perceived unhappiness with the marriage comes after the affair starts.

We struggle in life at times, everyone does. Your sister probably was and then the EA started. Slowly she starts to feel good (the addiction like quality described so well by Realityblows above) but at the same time she starts to blame her husband on why she was feeling bad before. She contrasts her emotional fantasy and dopamine hits from the affair with her previous struggles and the husband becomes the bad guy. This happens instead of taking responsibility for herself not feeling satisfied earlier.

Yes, sometimes people are not suited for each other. But in a situation like this it is highly likely her issues were not from her husband but within herself. And now the A is making her think her husband is to blame. It's insidious and makes people act delusionally.

I do not envy your situation but I would protect the family business.

This. You have knowledge of something that could potentially negatively affect your family's livelihood. I think you should tell your family from that standpoint and explain to your sister that you couldn't keep her secret at the possible expense of the entire family. You're either betraying one person's confidence by speaking or betraying the whole family by keeping silent.

Gently, the sooner the A ends, the sooner your sister can find happiness. There's no way she can go directly from here to happiness, though; she will have to go through immense pain. The longer she delays, the more pain she will have to endure.

She's blackmailing you to keep you from revealing her A. Your sitch will get worse and worse until you stop letting yourself be blackmailed.

I think you're in a lose-lose situation vis a vis your sister. If you keep her secret, she'll probably still be furious with you when the A comes to light. Your only hope for rebuilding your relationship is for her to change from betrayer to good partner. She's not guaranteed to make that change, and she certainly doesn't seem willing to do the necessary work now, so it's a slim hope - but IMO it's all you've got.

Meanwhile, outing the A

- makes your life easier, since it's a major effort to maintain a lie;

- gets your BIL more in touch with reality, and that will make healing easier ... with just a little luck, he'll see that his depression and anxiety have real causes; he'll see that he's not crazy; he'll eventually see he didn't fail - his W did;

- gives your sister's ap's W an opportunity to make choices about her own life based on info she probably doesn't have yet;

- gives your family an opportunity to protect their livelihoods.

There's no guarantee that outing the A will be welcomed by anyone, and there's no guarantee that anyone will say to themselves, 'I have effed up badly. I need to redeem myself.'

But I think it's virtually certain that keeping the secret will just make life worse for everyone - and keeping the secret may make the inevitable denoument even more devastating than it would be if you out the A on your next trip.

I'm sorry your sister has put you in this bind.

Remember, though, your sister has done the damage by cheating. Without her A, there'd be nothing to reveal.

[This message edited by SI Staff at 3:30 PM, Thursday, November 30th]

Yikes on bikes. I am so sorry you have been put in this position.

Few things. One - YOU are not "ruining you sister's life". She is doing that alllllll on her own. What she is doing is selfish, and self-indulgent, incredibly reckless, and so so damaging to her husband and the rest of her family, including YOU. What happens if she breaks this off and the employee that has access/control to your entire family's livelihoods decides to sue for sexual harassment? If he is her subordinate employee, he would have a case for this and could potentially take the company for a pretty penny in a lawsuit. But I am sure he wouldn't do that right, cus he is clearly a stand up guy (insert sarcastic eye roll).

Second - you are under absolutely NO obligation to keep this secret. None. Knowing what I know now after going through it and coming out the other side, if I were in your position I would inform your BIL, the ap's wife (OBS = other betrayed spouse), and your parents. I would do this immediately and I would NOT inform your sister you are doing so. She will be pissed no doubt. She will tell you you are disloyal and meddling and that you hate her and don't want her to be happy. She will say all of that and not see any irony in it.

Dovetailing with that - you are under no obligation to "support" her here either. You don't have to support this because what she is doing is flat-out WRONG and she knows it. She needs to grow up. Yes in a divorce she would have to split custody of her daughter and probably lose her house and it would be hard, but guess what? Those are natural consequences of the choices she has made. You are not creating any of those consequences if you expose this - she created those consequences when she decided (yes, decided because this is a conscious choice she is making) to engage in an affair.

And lastly, just my 0.02 but yes by keeping her secret you are enabling her. I will second that there's a lot of overlap with cheaters and addicts IMHO and as hard as it is, the only way 'out' for you is by not accepting or condoning her problematic behavior. You don't have to do this in a hateful/spiteful way, but you are under zero obligation to participate in her deception either.

The fact that you posted this shows me that you are a person with character and the fact that you feel very ill at ease here tells me that you also have a very strong moral compass. I am so sorry you have been put in this damnable spot - it is incredibly unfair of your sister to do this to you and is just one more relationship she's completely disregarding to get her jollies off. I am glad you found SI though, you'll get a lot of advice here. I would also send your BIL our way if you decide to tell him. This place absolutely saved my sanity after my ddays!

This is such a bad position your sister has put you in. What she is doing is extremely selfish. While I'm all for telling the OBW I feel that would be your BIL's choice to reveal that info. I would definitely have a talk with your parents first since it is their company and this could really cause big issues. Then maybe together, your parents and yourself can have an intervention with your sister. They will probably have to let the AP go from the company. It is possible at any time he can file a harassment suit whether he stays or leaves the company. Your sister has created a storm and I feel great empathy for you, her husband (the mental issues and depression this causes is emotional abuse), her children and your parents. Your sister either needs to end the A or get a D those are her 2 options. There is no future with this AP and her feelings are going to get hurt either way but she has brought that on herself and really needs therapy for this.

First off, sorry that you find yourself in the middle of all this. Infidelity is one of the most extreme forms of abuse that one human can inflict upon another. There is a whole book dedicated to this mantra, but "the body keeps score" is a very real thing and clearly your Brother-in-Law's (BIL) body and mind are showing the signs of trauma, he just doesn't know where the trauma is coming from.

Let me ask you a hypothetical question, rather than discovering your sister's affair, think about a role reversal, say your BIL was being physically abusive towards your sister or your niece, or both. Would you sit idly by while that abuse of your sister and beloved niece continues? Of course you wouldn't. You would be moving heaven and earth to make sure that your sister and niece get out of that toxic and abusive situation. I mean, it's your sister and niece, blood relatives that you would likely do just about anything to help and it's perfectly normal to have that reaction.

Now, I understand that your BIL is not blood family, but he is the father of your niece and your sister's husband. He is family nevertheless and for better or worse, because he is your niece's father, you and him will always be a part of each other's lives in some capacity. What you have discovered is that your sister is mentally and emotionally abusing your BIL and she is cheating on your niece in the process. Another hypothetical, say you found out not about your sister's affair but that she was poisoning her husband's food with arsenic and she was basically slowly killing him. Could you stand idly by while your sister slowly kills your brother-in-law and the father of your beloved niece? Of course you wouldn't sit by and not do anything. Any sane and empathetic person would be ringing all the alarm bells.

Here is the thing, to be the best of our knowledge, your sister isn't physically killing her husband with poison, but we know that she is undeniably killing him with mental and emotional abuse. Not to mention that since he is unaware that she is knocking boots with her boss, she is exposing her husband and the father of your niece to god only knows with regards to STDs and other nasty shit that Affair Partners (APs) are exposed to.

What you described to us here is that your sister is just a run of the mill cheater. Tongue-in-cheek around here, we refer to a mythical "Cheater's Handbook" because when you've crowdsourced as much wisdom on infidelity as we have here, with such a large data set (unfortunately infidelity is far more common than most folks realize), patterns emerge. One of the most common is "rewriting the marital history" which is exactly what your sister is doing. You and others didn't realize she was "unhappy" in her marriage because she was perfectly content, until this guy came into her life and started giving her the attention and fed her ego kibbles so that he could get into her pants. Suddenly, her husband is the enemy and the AP is the greatest thing ever and he makes her happy. That is like page 2 of the Cheater's Handbook and if you stick around here long enough you'll feel like you've read the whole "thing" cover to cover.

You said it best in your post, your BIL doesn't deserve this. I know that not everyone has close relationships with their in-laws, and my family has been no exception to that, but you are absolutely right, even if your BIL was a raging piece of crap and you didn't like him, he doesn't deserve any of this, because the reality is that no one deserves to be cheated on. Also, since you mentioned it, the Other Betrayed Spouse (OBS) is also being harmed. Her Wayward Husband (WH or in this case your sister's AP) is also directly in the line of fire for STDs and they have a 5 month old baby, which fits within the timeline that he was likely sleeping with your sister, so we can deduce that his Betrayed Wife (BW) is unaware of his infidelity. Not to burden you with further guilt on this, but your sister's behavior is destroying her family and the family of her AP. Not to mention the ties that this AP has to your family's business and how it could impact the lives of your parents.

I do not envy you or the position that knowing this information has put you in. I'm sorry to tell you this, but given the grave nature of this situation and how this AP is so enmeshed in the lives of your whole family, I feel that you are left with little choice but to do immediate and full disclosure to both your BIL (as the Betrayed Husband) and if you happen to know her, the OBS (AP's wife) as soon as possible. I'm saying that with the assumption that because the AP has worked with your family and you know about their young children, etc. that you may even know her or have her contact info. The point is that both Betrayed Spouses absolutely deserve to know what has been going on. At the very least, exposure to your BIL should be done immediately and allow him to decide what he wants to do about the OBS, but she should be told eventually. Hell, you can even point him here and the folks here will be more than glad to help him navigate this awful situation.

I mean, it sounds cliché, but think of the innocent children in all of this. One mother and one father are playing a very dangerous game of STD chicken that could devastate their whole families. If the AP's kids don't tickle your empathy, think about your niece. Do you really want to see your niece grow up in a toxic environment where her mother is abusive towards her father and her father has to seek medication and help from the constant abuse being inflicted upon him by the one person in his life who swore to never do such a thing? She is an innocent 2 year old girl caught in the crossfire here and this will absolutely fuck her up living in such a dysfunctional household. Your sister's excuses and whining to you about blackmail and her happiness is just a selfish, no good cheater and at this time she is being a negligent and terrible mother.

Your sister needs to learn the definition of blackmail, which involves withholding a secret in exchange for something of value (money, gifts, favors, etc). The person in the best position to blackmail her is, ironically, the man she's sleeping with because, as his subordinate, he could easily claim that she abused her position of power over him to extract a romantic/sexual relationship from him. As many a foolish male manager has learned after dipping his pen in the company ink, the OM might decide to turn on your sister if the relationship sours or if his wife finds out and feels he needs to cover his ass.

The other thing you need to consider is that it's very likely that other people at the company already know about the affair, especially if it's a small business. Generally, workplace affairs tend to be bad for morale because they erode employee trust, stoke division, and may lead to loss of talent or lawsuits, particularly if the OM is receiving special treatment because of his relationship with your sister.

Therefore, you really have no choice but to tell your family about the affair so that they can take action and protect themselves and the company from the potential fallout from your sister's behavior. And again, for all you know, some of your family members might have suspicions; you will just confirm what they already know.

The same goes for your brother-in-law (BIL). He pretty much knows that his wife is cheating on him, but your sister is mentally and emotionally abusing and manipulating him to the point where he feels like he feels like he's lost touch with reality and needs to medicate in order to cope with every day life. All you're doing by giving him the truth is confirming that he's not crazy, he's not paranoid, and he's not the source of his marital disharmony.

Another way to look at it is this: what would you do if you knew your sister was beating her husband to a pulp every night? Would you keep your mouth shut and pretend it wasn't happening? Or would you speak up and try to get him help? Because every time your sister leaves your BIL home alone with the kids because she has to "work late," every time she giggles at something on her phone before quickly tucking it away, every time he watches her get dolled up for the office, or sees lacy panties in the laundry that she's never worn for him, it's like she's punching him in the gut.

Last but not least, telling your BIL does not guarantee that he will divorce your sister on the spot. Although it seems counterintuitive, the first instinct most BSs have after learning about infidelity-- especially if children are involved-- isn't to immediately file for divorce, but to try to save their marriage at all costs. Whether or not your sister ends up divorced will most likely depend upon how she behaves after her husband learns the truth.

I deeply appreciate everyone's responses to this post, and am grateful for everyone's insight so far. It has given me a lot to think about. I still have some questions, and am not sure what I am going to do yet.

I think exposing her affair is the right answer. I just don't know how to do it. I see my family on average once a year (active duty stationed overseas). This is the first time I've been able to spend a Christmas with them in over 8 years. They already see me as the 'prodigal child' because I'm the only one that does not work for the family business, and did not stay in my hometown. If I expose my sister's affair and hurt all these people, I fear I may not be welcomed the same as I was before. Especially around the holidays. I don't want my family to dread my visits. I fear they will see me as the bringer of bad news and drama.

Should I give my sister a timeline to come clean herself so she doesn't look so much like the 'bad guy' when it comes out? My father is the owner of the company and will be furious at her because he has always feared infidelity from my mother (whom has been nothing but loyal to him).

If I do give her a timeline, how long should that be? One week? One month? Six months?

I have no definite proof of her affair. But she did verbally confess to me because I called her out on her strange behavior. So if I tell my brother in law, he will have to say that he got the information from me. This would be fine, but my sister can be vengeful, and our relationship would never recover.

I agree that infidelity is like a drug. I had not made that connection before, and has given me a valuable paradigm shift. Her language and disregard for other people because of her feelings has made that clear.

I don't want to lose my sister. I don't want to hurt my family. I feel like I'm stuck trying to make the right decision in a wrong situation.

For anyone who has been the wayward spouse, how would you want a third party to treat you in this situation? Is there any outcome of having a good relationship with my sister moving forward and exposing her affair?

Another unfortunate thing is my family lives in a very small town. Any word of this will spread like wildfire and greatly harm my sister's reputation. I know she is doing this to herself, but I still want to protect her from the consequences she has made for herself.

any continued advice, personal experience, hindsight, or any additions are greatly welcomed and appreciated.

You can’t leave this in her hands. She will turn this on you and tell her husband and family you are a crazy troublemaker. Her H and the family will be prepared for your disclosure and it will make you look worse.

You need to control the narrative, let the family know this breaks your heart, but you have to do the right thing to get this out before it gets worse.

She became very upset and said I was blackmailing her. That I was taking away the only thing in life that made her happy. She told me that the only thing she wants is happiness, and the only person that brings that to her is her co-worker.

Sounds to me like the person who's being blackmailed here is YOU.. emotional blackmail. Your sister has made you into an unwilling accomplice while she makes a stooge out of her husband and if you stand up for yourself and refuse that role, she's making you out to be the bad guy. Well, you are NOT "the bad guy" in this. You're just a decent human being who has stumbled across an inconvenient truth.

Cheaters try to treat the truth like it's the problem, as if discovery were the sin rather than the action which precedes it. When you put the focus on the action, the path becomes clearer.

I think, if it were me, I'd reassure my sister that I loved her very much but that she was NOT going to make me culpable in her poor choices. I'd give her a short, limited time to make corrections, and if she failed to do so, I'd do it for her. The owners of the business need to know what potential liability they face, and IMO, the BH needs to have his agency returned to him. What "corrections" looks like though is really up to you and depends upon what your personal values are. You have to live with your choices. Right now, your sister is trying to dictate your choices to you, and it's okay to set boundaries and not allow anyone else to put you into a position where you have regrets.

First and foremost, this is going to come out... one way or the other. There's no just no way in hell that a workplace affair at a family-owned business in a small town is going to remain a secret forever. As I said in my last post, I can almost guarantee that at least someone other than you knows about it. So you can either be the bearer of bad news in a way that helps your family mitigate the damage her affair will cause... or you can wait for this bomb to detonate.

Second, you will be damned if you open your mouth and damned if you stay silent. When (not if) the affair is eventually exposed, you will be blamed for knowing about it but saying nothing. Or a lot of people will be angry at you for speaking up. If you're going to be the target of ire no matter what you do, you might as well do the right thing.

Third, as you as you get a chance, call your father and let him know. Explain to him the dilemma as you laid it out here and tell him that this is really hard to do, but you needed him to tell the truth. And then you let him to figure out what to do next from a company perspective. The next call is to her husband. That will be trickier. There's no nice way to tell someone their spouse is cheating. But just be factual, compassionate, and to the point.

Fourth, don't give your sister any lead time or prior warning. All she's going to do is try to cover her tracks and paint you to be the villain. Unfortunately, there's really nothing you can do to prevent damage to your relationship going forward. Perhaps, if there's a shred of decency within her, she will eventually realize that you gave her an opportunity to save herself and her marriage. But if she's a selfish narcissist through and through, then at least you did right by the rest of your family, especially your BIL.

You could wait until after your visit, inform your brother in law anonymously, explaining that she will cover it up if he confronts her right away. Maybe even make chat gpt write the message, so it's generic. This will minimize the impact on you.