What to do for my younger brother?

She sounds like she enjoys the drama of making your bro jealous and getting him into fights over her, some sort of lame ego trip, but this, and more, is the world of dating high maintainance girls. My advice to you would be to make him see these things that make her a bad match for him, as he may not see them himself due to certain emotions that may be involved. Perhaps he doesn't see her the same way others do, if you can change his perspective to your own, maybe he will break up with her. Good luck :)