There is a 12-year backstory to this, but I consider my brother's wife to be toxic, based on her horrible behaviour to my mum over the years and also to me on occasion.
They've been through 3 cycles of things getting better (generally after my mum has made constructive efforts to repair the relationship) but then SIL goes bad again for little or no apparent reason. SIL has made it impossible for my mum to maintain loving relationships with their DC - at the moment Mum doesn't see SIL, and my brother brings the DC to her place every couple of months (they live 5 mins away). My mum is a wonderful grandmother to my children, and would have been to theirs if given a chance.
I find the whole thing extremely upsetting and have also been on the receiving end of her toxic behaviour myself. I now have a kind of physiological reaction to seeing her where I can't be normal and polite beyond saying hello, and just want to run away and vomit.
I live overseas and only go home every 2 years, and the stress of seeing her ruins my trip every time. I've tried various approaches (reasoning with her, saying I don't want to be in the same place as her and my mum at the same time, reconciliations when things were better with my mum etc) until I finally decided not to see her any more.
The problem is DB, who wants my family and his to play happy families! I thought he'd taken my position on board but on my last trip he assumed we would all get together and I ended up agreeing to apologise for a minor factual mistake I made 2 years ago and to suggest we put things aside for a couple of hours and see each other. SIL wasn't going to reply until I told DB she needed to, and then was still holding onto her grudge and saying she'd been judged unfairly.
At that point I decided (again!) not to see her, and didn't, but there is an expectation that we might see each other on my next visit. But afterwards I got depressed for a couple of weeks and I realise that I really don't want to. My parents (divorced) both agree, as my negative feelings about SIL mean I'm not in a good state to enjoy my time with them when I go home.
AIBU to insist that I don't want contact with her? DB makes me feel like a dreadful person for not wanting to see SIL, and he thinks it's unreasonable for me to support my mum ('two against one'). Personally, I don't understand why SIL insists on seeing me when she clearly doesn't like me any more than I like her, but my feeling is that it's some kind of power game.