Howdy, this is Gunna be kind if long, hoping someone can give me some insight on this situation

My brother is 19, in college, and is dating a girl.. who already has a boyfriend..

My brother knows about the other guy but doesn't care. The other guy however, doesn't know his girlfriend is cheating on him. If he knew, he would leave her. He has even told her that because he was suspicious but she denied that she was cheating. The girl literally goes to her boyfriends house, has sex with him, then leaves and has sex with my brother. She talks shit on her boyfriend to my brother constantly, and says she doesn't even like him but at the same time she doesn't want their relationship to end. My parents and I do not understand what is wrong with her or my brother. My father's first marriage ended because his wife cheated on him, he walked in on it. So I feel like that would effect how my brother feels about cheating but he does not care at all. He could find other girls to sleep with, why does he have to sleep with one who is already taken? I've asked him this and he could not give me a reason, we've argued over this several times but it still bothers me and makes me question my own relationship. The more I see other people cheating the less secure I feel being with someone, does that make sense?

Anyway, what the hell is wrong with her?!? Why does she need to have 2 boys? Possibly even more than that? Why do cheaters always want it all, they want to cheat but don't want to lose their original mate, it makes no sense!