Coparenting is dysfunctional.. help?

My boyfriends ex is his daughters mom. She’s married and has two other kids by her husband. Knowing all of this, I thought there wouldn’t be any problems.. right? Well, wrong. She and I get along but now it’s to the point where we’re close enough to talk about our personal lives. Her husband has domestic abuse charges on him. And a child abuse charge! (One of their kids). She acts like this isn’t a huge deal. My boyfriend has said in the past that he doesn’t want his daughter going over there because of her husband (he has full custody of his daughter). I am very pro-mom and didn’t know the full extent of the story until now. She even told me her husband has fantasized about a foursome between all of us, and talks about how pretty I am, asked her how big she thinks my tits are, just weird shit! I don’t see how or why she’s with him. My boyfriend has told me that she is also crazy too, that she used to hit him all of the time when they were together and lies a lot. So to be as simple as possible, we’re dealing with a really dysfunctional couple and my boyfriend wants to cut contact with them. Her mom technically only has supervised visitation. But we’ve allowed her to go over there for a few nights a week. All of a sudden their daughter comes home to us, tells us she’s afraid of her husband and doesn’t ever want to see him and mommy again. Then they tell us that he smacked her across the mouth as a form of punishment. (Which I am against personally). His daughter mom and I were talking about it and she even admitted he did it more than once and “might’ve been a little rough” and acts like it doesn’t phase her. I would go bat shit if I heard someone laid their hands on my son! (I have a baby from another relationship).

So my boyfriend told them she was no longer coming over. Then what happens... her husband text me separately and tell me things that my boyfriend has said about me. Apparently mean things. He goes into detail and it sounds believable but even his daughters mom said it’s a lie. I feel stupid for not knowing what to believe because I SHOULD believe my boyfriend, however some of these things were a bit personal and I could maybe see my boyfriend saying them? One example is that his other ex (not his daughter mom) has a hot body compared to me or whatever. But my boyfriend says he never has said that. It’s just so dysfunctional. I’m going to block both her husband and her because i have no obligation to talk to them. But I’m wondering if it were you, would you leave this type of relationship knowing it’s something you’ll forever have to deal with plus bringing your own child into it? I need help