Hi,
I usually a very understanding person and try to see all sides, but this time , I'm not going to mince words. At first I was going to stay she's a ***** but as I read on - I'll add slut to that too. I know this doesn't help[ but it feels good to let it out. I doubt if they'll be happy for long anyway. Good thing your mom doesn't have to put up with him anymore.
Make sure your mom gets any support for her/you and other siblings. Don't worry about it and don't let her get away with being rude. I normally don't swear, but I hate people like this. TC