ohhh yea no i’d honestly have done more than just slapped her. you’re not the a-hole, you were just defending your mother and sister. the people who are mad at you it’s probably just misdirected bc of the situation. if anything i think your sibling who is married to your SIL should have stood up for your family. if it was meant to be playful between your SIL and sister that’s one thing but it seems like it was more so an action to shut her up which is unacceptable.
honestly if he’s defending her and not his own sister who got slapped that’s a huge problem and you can see where his priorities lie. your SIL’s family is probably mad because either slapping was normalized in their family and according to them “she didn’t do anything” so why slap her? OR they aren’t being given the whole story. either way they’re all grown adults who are mad at you for doing the right thing. you have nothing you can do and if they’re going to be that way then you have every-
-right to cut contact, limit contact or request an apology. you do not deserve to be treated that way when in the end you defended your family and your brother did not. he’s more worried about someone who hurt his own sister and yelled at his own mother than those two women. HIS behavior along with hers are unacceptable and they deserve to know how mad you are. you’re not in the wrong.