I estranged one if my sisters. My parents were blaming me and saying I was breaking the family apart.

I reminded them what she did to me and how they'd feel if she'd treated them the same way. Every time they tried to bring her up, I told them I'm not talking about her, I don't want to hear about her, and that's never gonna change. I stopped conversation and left. I don't attend any events that she might be at. I made it pretty clear that it's not happening.

Eventually they accepted that our relationship was irreparably damaged. I had to become unreasonably adverse to even hearing her name before they realised that it's a lost cause.

Your sister might have to dig her heels in to show your mother that she isn't going to be swayed. Your mother is pushing her, probably because it's easier than telling your brother that he's been a cunt and his family doesn't respect him anymore, and that he's the only one who broke it apart. Probably because your brother is very unreasonable. Probably because he is spoiled. Probably because your sister is the nice one.

Support her, and let your mother make the decision on her own. It might hurt that she won't attend the wedding, but that is her choice. Enjoy the day to its fullest.

But you'll both have to set hard boundaries before people begin to respect them.