What you describe is truly a very lamentable situation. I'm very sorry that this is all you have left of family. It sure doesn't sound like there's a whole lot you can do about it, which must be an ongoing deep disappointment.

You can let your sister know you love her and that you are there for her. Then you have to step back from her because you can't allow her to act as a channel of abuse coming from her husband toward you. She has opted to accept this abusive marriage, even though it may have cost her a son. That husband sounds vile. You have every right to disengage from toxic interaction. That may mean having little contact with your sister, which is very sad. It is something to grieve over.

I hope you have others in your life. Try and cultivate warm friendships. There will always be a void where closeness to family should have been. You have to invest in other relationships.

I've known some difficulty in my own family due to resentments over who inherited what. I too have a brother-in-law who seems to manipulate my sister at times. When I visit them, I feel he just tolerates my presence. None of this is anywhere near as bad as you describe. But I know how disturbed I've felt, just getting a slight taste of that alienation. My sister does stay in contact with me and our relationship is reasonably warm. She's not being abused. You must wish you could help your sister to escape what she puts up with. However, she chooses to stay, and that's that.