Oh dear, this is a very tricky one. I think your B-I-L has left you in a very awkward position, by telling you, yet not confronting your sister - his wife.
Why can’t he speak to her about it? Is it because he was snooping where he should not, and to say anything would reveal that he didn’t trust her, and was checking up on her?
I’ve never been married, but I must say I can’t really imagine finding letters of that nature from a partner, and NOT wanting to address it with them.
Is he scared that mentioning it might push her over the edge or something, or just embarrassed it would come out he hasn’t respected her privacy?
If it was on a shared computer, and not password protected or anything, is it possible she intended him to find them, and it was some kind of cry for help?
Of course, you are right that some people use writing as a release, and may have no intention at all of acting on their thoughts. Perhaps your sister just found it empowering to think she retains the option, even if she has no intention of actually using it? I was once told by a psychologist that many people (even those deemed physically and mentally “normal”, by conventional standards) have at least thought about suicide in an abstract way, but without any realistic intent.
Might that have been all your sister was doing? Exploring an abstract idea, but with no intention to go ahead, or that the letters would ever be found?
I’m sorry to have to ask (and you might not know anyway) but did the letters make any explicit reference to self harm, or could it be that your sister simply fears she may die of natural causes? Dying as a direct result of MS is not very common, but it’s probably something even the most mildly affected of us have thought about occasionally. If your sister has had very pronounced deterioration in just a couple of years, is she perhaps fearing, rather than intending, that something might happen to her?
Although I would feel tempted to approach your sister, and confide what you know, I appreciate that risks causing a rift between her and your brother-in-law, as it reveals that he not only read “private” documents of hers, but then shared the content with another family member, not her.
What is your relationship with him like? Is it good enough that you could urge him to confess to your sister that he found the letters, and tell her he is worried?
I don’t think pretending he didn’t see them is really the answer - especially with something so potentially serious. If she was/is serious, he could miss a vital opportunity to intervene.
I wonder if speaking to a confidential helpline could give you any pointers. I was going to suggest The Samaritans (even though it’s not for you), but I have a feeling they deliberately refrain from direct advice, as part of their philosophy. However, they might be able to pass on details of organisations who do advise. Otherwise, you might try a mental health charity like Mind, or perhaps the MS Society’s own free helpline (number at the top of the page).
I do not think it advisable simply to ignore that someone is - or may be - contemplating self-harm, but this needs a delicate touch, so you and your brother-in-law are not seen as in cahoots against your sister.
I think the initiative really lies with your brother-in-law, but you cannot force him to raise it with her, if he’d rather stick his head in the sand. This is where a specialist organisation might have some ideas.
I’m sorry I don’t have any other suggestions. I really hope it was just a form of self-therapy for your sister - exploring ideas that are far from reality.
Just one more thought: do you think it may have been connected with Christmas? Many people, both well and ill, find the run-up to Christmas particularly stressful, because there’s so much emphasis on everything having to be magical and perfect. Many people’s reality falls well short of that, especially if we can’t do things we’ve always done, or that the ads keep telling us we’re meant to be doing. Might it have been related to that?