This is more about the man than the partner but titled it this way for shorthand.
I'm a sibling in this situation but imagine many parents have experience similar with sons - looking for advice.
We used to be a very tightknit loving family. My brother was with and then married a woman who we didn't think was right for him and we worried about him a great deal. For many years it was an acceptable polite relationship but over time he became more and more resentful and sullen with the rest of the family. With hindsight, I wonder if in his naiveity he actually told her what we thought which sullied it from the off - so she didn't want to see us and had her guard up? I have no basis to think this just idle wondering.
A few years ago our mother died and this seems to have brought out another side to him.The resentment and dwelling on past 'perceived' issues has just got greater and greater from him. The situation has deteriorated and he is barely speaking to any of us. It's sort of become a Harry & Meghan/ Brooklyn Beckham & Nicola Peltz situation where he now speaks badly of all of us all the time and blames it all on bad treatment of his wife (which is just not true).
Me and my sister both miss our close relationship with our brother which went years ago. My dad is devastated but is of that stiff upper lip 'let him get on with' generation. I think when he dies it will be the end of contact with our brother.
So my question is, has anyone got any positive stories about a happy ending in this situation? is it recoverable? or advice about what to do? do I have to resign myself to the sadness of that relationship having died. Why do men do this? I read lots of stories of this sort on here but it is almost never about a daughter/sister rupture due to their partner.