What can I do?

Okay so about a month or so again my dad slapped my brother upside his head really head bc he got a bandage after he told him he couldn’t have one, and then tonight he hit in again and I really don’t know what to do anymore I’ve told my mom and all she’s done is talk to him. Call me crazy but that’s not good enough for me my brother is only 5 he shouldn’t hit him that hard or at all if I’m being honest I wanna do something but I just don’t know what? I’m only 17 (he’s slapped me before too but I was 16) I have 2 younger sisters and it’s really scary having to worry about there safety idk I’m just really having a hard time trying to figure out what I can do since my mom won’t do anything and by do anything I mean leave.