Not trying to be rude here and I completely agree with the other comments, however I do want to ask how is it actually hurting you? In the grand scheme of life is this girl that you have an obvious bias for actually doing anything wrong to you besides you just don’t like her because of XYZ reasons?

I’m not saying how you’re feeling is wrong but there are different sides to everything.

As for your brother, he’s made his choice. Maybe he will realize before he gets too badly hurt how she is. It’s not your choice to make, all you can do is advise him but it’s up to him to take that advice or not.

The only thing that you can change in life without a shadow of a doubt is yourself.

I do hope that everything works out in the end though.