This is not about me but I still want some advice?

One of my younger brothers has been with his girlfriend for over two years and everything was good. Lately they fight too much, they break up and make up all the time. I don't think this is a healthy relationship. One of my older brothers who is soon marrying this girl's older sister, has said to have seen her with another guy. SHe claims he's "just a friend" but do "friends" really hang out every day, stay outside the house and inside the car late at night at like 2 a.m and just talk? the thing is that my brother already told the other one about what he's seen but this stupid girl denies it and my brother believes her. I know that no one should get in the middle because it's all up to my brother to decide and open his eyes but he's one of my baby brothers, I can't stand to watch this girl make a fool of him. I don't know how to help him open his eyes. It's like he's hypnotized or something. They had broken up but suddendly she took him back before prom,, isn't that a coinsidence? ofcouse my brother paid all the expensess and now after graduation, she fights with him again... I think she's using him. I would really like some advice that would help me calm down cus I feel like next time I see this girl I'ma beat her up and that's not how I solve problems but if someone messes with my family I do what I gotta do to protect them. Someone please help me before I do something stupid. Thanx :)