While I agree, I do think it also depends on the sister’s age.

This screams “only child.” But as the other said it definitely depends on her age and also the issue is the mom being physically abusive and the sister enjoying seeing him get in trouble. Or did you get so heated over making this comment all logic got thrown out the window so you could scold him?

Perhaps the sister is 12? In that case I find snitching justified.

Given Mom’s reaction, I suspect that OP has a history of badmouthing sister. Either that or Sister is the “favorite child” for some reason. Unless sister is a minor and boyfriend is an adult, it would have been far better for OP to first talk to sister about the danger she might be in if that was the case - As opposed to simple jealousy or envy. But OP’s post said nothing that implied that. In a family you have to learn to get along with your siblings otherwise they can make each other’s lives miserable. And most parents really don’t want to hear a constant stream of sniping by one sibling about another. Of course it was wrong for Mom to slap OP for telling her something she obviously didn’t want to hear - Very poor parenting! But the reaction says there is another story going on here. Either Sister is the “good child” who can do no wrong or this is just one more time OP has tried to get sister into trouble.

I can't believe this doesn't have more upvotes and everyone seems perfectly fine that this person got slapped by their mother! That's not ok and there's no excuse for her violence.