I have a few questions/issues simply to better understand what’s going on. They aren’t necessarily connected, so I’m just putting them out there...
To begin with though – I don’t necessarily see infidelity as we usually define it on this site. Without knowing more about the relationship between W and BIL (then "only" second cousin) then it is possible for them to have gone on to have a "normal" non-romantic and non-emotional (beyond close family) relationship. There is a lot of maturity that takes place in the brain, in social skills and all that from 15-25, so their ongoing relationship COULD be "normal" and non-impacting on your marriage.
But then – for all we know it could have been a full-on EA or whatever. We don’t know, and I generally don’t decide my future actions on what I don’t know.
Not saying I don’t see issues in the marriage and I do understand your frustrations and concerns.
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What other letters were in that box? I find it... well... strange... that sexually charged letters between 18-year-old W (although not wife at the time) and a 16 year old boy – second cousin, and so close that he’s adopted 50 years later – are being stored and have sort-of prime-of-place at the top of a box. Since they didn’t approve of the relationship then why did they keep them for all these years? Why at the top? The general content/gist of the other letters might offer some clue.
Were it your wife storing the letters – I would imagine them hidden at the bottom. If her – any indication that she’s been going through them in the last decades? If her – then why allow you to clean out the house had she known of their existence and prominence?
Was it your late in-laws... why? It’s strange that they hang on to something they disapproved of, was probably shameful to them. Something that borders on the Romeo-and-Juliet laws in most states (although different times back in the 1970s).
If this relationship was significant enough for them to store the letters – why adopt him?
It might help if we knew who packed that box and stored it for all those years – your W or the in-laws.
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On this site we often talk about how the WS rewrites marital history. I’m wondering if you might be doing that too... even as a BS.
You state that out of a 50 year marriage the last 30 have been rough, with the last 10 especially bad and the last six months terrible.
Yet you also talk about traveling the country together, visiting grandkids, motorbikes... all in a positive tone. Maybe you did all that in the first 20 years, but this indicates that there were some positives in the marriage.
Then the drive all that way with her for her cancer treatment. Indicates care.
Not saying the last decades have been great, but have they really been bad?
Only you can answer that.
I would suggest you build your decision regarding your future on that. If this has truly been a miserable existence for 30 years, definitely seriously consider separating your lives. It’s hard to break behavioral habits, and changing a 30 year old pattern near impossible.
If it’s been OK, but some decline recently, seriously consider separating your lives BUT just as seriously consider what could be done to get the marriage back on track. IMHO that is usually done with clear – mutual – communication.
The love-letter issue? Communications. No – she can’t shove this under a rug, but at the same time you need to be willing to hear whatever she says and – if plausible enough – accept it as truth.
At your age (and I’m not really too far behind) keep in mind that your spouse is the person you are most likely to share whatever bad news comes along in the future. My wife is the one that got the "honor" of checking if it was a pimple or a hemorrhoid. She’s going to be the one who get’s to choose when to flip the switch if I ever go on life-support (and vice-versa). With all your history there shouldn’t be anything off limits to discuss. But with your age it should be done in a productive manner.
I think your best bet is to make it clear to her how this is damaging you, and how it’s really making you very seriously consider if your marriage has been fake and if it really has any future. Make it clear you want to talk this thing through – not to find villains or assign blame – but to find resolution.
If she doesn’t contribute to that... well... it’s been a good run.
I encourage you in the strongest way to tackle this as soon as possible. It will either lead to an inevitable termination of the present marriage – shortening your time to a better life – or to a healthier and happier marriage – shortening your time to a better life. All delaying will do is keep you in some form of misery.
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I want to briefly address one issue: This site doesn’t have a pro-r slant. I don’t presume to talk for the site, but simply by knowing it was founded by a couple that reconciled and was mostly held together (until her untimely loss to cancer) by a wayward wife, and by knowing a part of staff over the years have been former wayward posters, and seeing there is a Reconciliation forum then this site clearly believes reconciliation is a possibility. Just like the Divorce Forum and the New Beginnings indicate this site acknowledges that R isn’t the only path.
For some simply thinking or even suggesting a couple can reconcile is sacrilege. I guess that for those fanatics this site is heresy.