i think my sister is being abused, need advice what to do

Hey guys. My first post here.

I have a problem which is giving me quite a headache lately.
My father was a full blown narcissist. He is gone, and now my brother is acting like him.
He is exactly the same. Manipulative, abusive and overall problematic to everyone around.

I noticed he is being abusive to our younger sister. He is 21 years old. She is 13 years old.
I noticed some strange behavior.
What I mean by that is that he is insulting her, touching her where she doesn't like, and ignoring her when she confronts him. Always taking away her stuff, so she began hiding everything.

I attack him every time when I notice it, but then he goes in a rage and she protects him so she might be in a denial. She is also having nightmares lately so I have a feeling she is in fear subconsciously.

I don't want her to live like that.
My protective instinct kicks in every time but I don't want to act only on emotions if there are better solutions.

Any advice is welcome.

Thanks
Stormseeker