Mom hates sister's husband, becomes ugly talking about him. She wants my sister to take care of her. She cannot live alone due to her memory problems. It's the same conversation at every visit and phone call. She becomes so agitated and hateful I have to leave.
It's too cruel to tell her she has to stay where she is forever so I try to change the subject - you can't explain something that firstly they don't want to hear and secondly they cannot remember
Let her ramble on about her plans, and nod and smile and give non-committal answers, like "we'll see" and "maybe" and "that's something to think about." Add questions about how she thinks this might work, and what she would expect to be better about a change in her environment.
When your mother gets on to the subject of your BIL, and you find it too hard to listen to slanders and vitriol, try to change the subject with "well, that's enough about him, let's talk about XYZ." But has she always disliked him, or is this a change in her attitude that could be part of her dementia?
It sounds as though mom is trying to save sis from BIL. Don't argue. If you can't redirect, leave.
My cousin was under the believe that she could live alone, WITH the help of her neighbors checking in time to time. It was absurd, but I just said that we had to do what the doctor wanted for the time being and of course, the doctor said she could NOT live alone. It wasn't even a close call. She eventually did heed the doctor's advice.
"end of life" place not home, etc. My husband, who usually stays calm, even got to the point of total frustration. We finally decided to only make 1/2 hour visits and if they seem to go well...extend or leave as necessary. I love my mom, but when you can't redirect and they are becoming agitated and you are upset, then it is not healthy for anyone. As you say...Dementia will win the argument everytime.