What would it take for you to like your brother if he beats up your "boyfriend"?

I can't say I've been in your situation, but I look out for my younger siblings too as an older brother. I think it's best just to tell your side of the story to your sister, and let her know what she said. Then she has to make up her mind. No matter how you wish it was different, it's her life, so keep an eye on her and stay in the background until needed.