Just want opinions

I have a sister who is married to this guy and has 2 younger kids with him and then she has a older daughter from another guy. Anyways my brother in law is a piece of shit. I mean he's just beyond worthless for so many reasons. However the biggest reason why is because he's constantly treating my niece the one who isnt his like crap. She's 13 and constantly gets verbally abused by him. It has gotten so bad to the point she went to the school counselor and told her how she's scared to go to high school because he always stresses her out and she feels like she won't make it. Its truly just heartbreaking how she has to go through all this and I'm the only one in my family who speaks up about it. My sister doesn't see the problem and continues to stay with him and brag about how much she loves him even though he has at times physically abused her along with everyday verbal abuse. The counselor even called .y sister and it's like my sister doesn't see what's wrong or anything. But the counselor said she can't really report the abuse because it's harder to prove verbal abuse. But I'm sick and tired of letting this scum bag get away with it. I've offered my niece to stay with us but she doesn't want to hurt her mom and would rather stay with her grandma (my mom) anyways. But I'm at a lost on what to do anymore because I'm tired of letting this guy get away with the shit he does.