B.R.
S.,
Did I understand this correctly that 28 years ago, after you divorced, your best friend's sister in law (who was your husband's sister?) and now, after 28 years, there is friction? There is probably more going on that you know about and more that anyone will admit to, but I would say you should stay gracious and respect their wishes. I bet things will turn around again eventually and you will have your friend back after a good talk. It's best that you don't create more friction to give them more reasons to blame you for whatever. People might end up saying things they'll regret. WOrds can't be unsaid.
I can feel the hurt in your words, but you sound like a reasonable, calm person, so stay gracious and patient. :)
(By the way, if your ex recently remarried, it is quite possible that the new wife is the reason this is all coming up now after so long. Give her time. She probably feels threatened, especially if you have had an amicable relationship with your ex for all these years.)