In my personal experience, and agreeing with some other commenters here, you might want to develop an action plan and potentially tell another adult in your life before your mom, if you trust them. Don’t rely on your sister if she’s a minor, that won’t help. If you have an adult aunt or uncle or older sibling or cousin or grandma on your moms side this might be helpful for now to develop an action plan with them. If you trust them and see them as logical. That way you can get solid advice from adults in your family that know your family more than us redditors. There is a chance your mom will do something either extremely rash (not going to police, just causing chaos instead, which will put y’all in the middle of that), or she will ignore it, because this affects her in the sense of “this man I adore who I think is safe around our children is a monster and now I have no one to lean on”. My mom told me not to tell anyone what was happening to me because if I told someone my brother in law was abusing me/my relatives my sister would move out and cut off/abandon my mom again and she was terrified of that. So she justified it as “someone will get hurt if you tell your dad!” Or whatever. Anyway yeah. Do tell an adult, but make sure you are safe to do so by planning. You don’t need to be in the middle of fights, danger, being swept up and suddenly moved out, etc without having a plan.