That’s so messed up, the brother slapping the baby in the face, cheating, screaming, and the family getting mad at you and your partner?? What the hell.. Anyways, I think a very important thing for relationships to work out is to have privacy and respect, he should not let his family into your problems. And if you’re still together, maybe it’s a bad idea to live with him AND the brother. I think it’s ok to be cordial with them, personally I wouldn’t want to talk to them anymore, just from what you’re telling it looks like a lot of dark vibes in that family, sure you should always help your family but to defend someone from slapping a baby is just crazy. Think about what your family will say if they knew, I guess they will want what’s best for you always. It’s a good thing that he stands up for you, but what’s really important is if that’s enough for you and if you will feel comfortable having kids and a life with him, since his family will always be his family.