There is nothing to do. SIL is, slowly, very very slowly, realising that he is a total waste of space. I suspect that she's finally at the point where if it wasn't for Christmas, she'd be breaking up with him and that at some point next year she absolutely will.
In the meantime though, while we've all lived with how selfish, manipulative, controlling and lazy he is for years and years, it's all escalating as because she is finally at the end of her tether she has stopped trying. Cue him accusing her of all sorts of things, incoherent messages to her and the rest of us (including her friends). He can't even bring himself to be polite to my Dc when they go into the house.
I'm posting in chat because there is no solution. I just need to rant a little. I hate him. I hate him. I hate him. It breaks my heart to see SIL so unhappy. I worry about our nephews and how the atmosphere in their household is impacting them. I can't believe how his behaviour over the last 12 years has turned SIL into a person who questions her most simple reactions.
Perhaps what I hate the most is that first we missed the warning signals. Then, when we did see them, long ago, we couldn't do anything. Maybe we should have tried harder? But he was clever and I think none of us fully realised how much power over her he'd managed to implement. We thought it wasn't great, but I will never forgive myself for not sitting her down 8 years ago and saying, "this is totally not okay". Even though I know that she would have justified his behaviour and, if anything, it would have allowed him to isolate her from us (which is the one thing he tried to do but was not successful at).
I really really hate him. And I honestly am not sure that there is anyone I have ever hated before. I am dreading spending christmas with him, even though we are spending christmas with them because we knew if we didn't she'd be left with him alone and who knew what would happen.