Is it normal for a 14 year old to date an 11 year old?

My brother is turning 15 in December. His girlfriend is 11 and he's a total dumbass. My Mom finds so many things wrong with it and my Dad doesn't know but since he doesn't like the girl so it'd bad If he found out. Now my brother is a total dumbass so he'e oblivious to everything. Now she's my sisters best friend so she has an excuse to come here every day. Apparantly she's two timing him but he already knows. He's a complete moron. She begs him to go out with he and then kiss her. Guess what? Of course does. She a really flirt too she likes a guy and the moment she finds someone better she dumps the old boyfriend. She tried to get with me too but failed. She wants to do it with him. The dumbass probably will if she begs him. We found a condom the other day and so my dumbass sister decides to tell her about it. Of course I tell her to shut up and she has a big mouth. Does anyone find anything remotely wrong with this? What should I do?