@Magmum13 Here are a few lessons learned from my situation with my SIL.
A couple of years ago we learned that my SIL was apparently very upset with us over something we had done, and that was the reason why we hadn't heard from her and BIL recently. We did not understand why she was so upset about something we considered trivial, but we reached out on multiple occasions to try to talk about it so we could reconcile and move on. She completely refused to engage with us, preferring to remain completely no contact with us. On the rare occasions we managed to get my BIL to talk about it with us, he just repeated her stance and said there was nothing to be done, even though he could not articulate why what we had done was actually a problem. The situation has continued to escalate and in recent months my BIL has been largely no contact with my PIL as well, who are no longer seeing their grandchildren as a result.
If I could have my time again, this is what I would do. I would send a message to my SIL and BIL together, saying I was very sorry she felt the way she did, and that it had not been in any way our intention to upset her. I would then grey rock her as much as possible, doing absolutely nothing to aggravate her, being pleasantly boring when we were required to be in the same place, and essentially non-existent otherwise. And I would try to maintain communication with my BIL as best as I could.
You cannot negotiate with terrorists.
You cannot change the person who is the aggressor.
And you cannot resolve conflicts in this situation, because that is not the point.
I no longer believe that my SIL was actually upset by the thing she claims to be upset about. And that is why our attempts to talk to her about it were completely stonewalled. Because she did not want an apology, she did not want to reconcile and move on. It was a completely manufactured conflict which she always intended to maintain over the long term. The entire point was to go no contact with us and in doing so, isolate her husband from his family so that she could have total control over him.
I feel in retrospect that we played into her hands by trying to fix it and getting angry when she wouldn't engage.
If I had apologised for inadvertently upsetting her and then just engaged with her as little as possible and led an entirely blameless life from that point onwards, she would no doubt have continued to rant and rave about how my husband and I are the devil incarnate, but maybe somewhere my BIL would have been thinking, "But they apologised for upsetting you and since then they have done absolutely nothing to you. Why are you still angry?"
Unfortunately, because the situation has escalated, she now has more to work with when trying to convince him that we are evil people and that none of them should ever see us again. And because my parents in law eventually stuck up for us, she has accused them of favouritism and now she won't see them either.
There is nothing you can do with people like that. All you can do is grey rock them as much as possible and try to keep lines of communication open with the person or people they are abusing.