Unless he decides to break up with his girlfriend because of his cheating you should probably leave him alone until he becomes available, it's best to just not get involved.
Why are you dating my girlfriend?
Break up with her!
Of course
No that is bullshift, if the girl wants to break up then she is stupid.
Yes you should and ask her why she wants to break up.
Break up.
be honest with her
If a boy has a girlfriend then he is considered taking and you should leave him alone and later he may break up with his girlfriend.
You should trust your girlfriend and if she loves you she will ignore her brother providing you are treating her with respect. If she does not stand up for you and her brother continues to try to break you up she should either stand up to him or, if a minor she should talk to her parents about what her brother is up to.
break up u hate her
The feelings you have for your girlfriend should be strong. If you don't have these feelings you need to break up. There is someone for everyone.