You don't mention your ages. Let me ask you something. Let's say you were an artist, the best one in your school. Heck, the best one in your town. Now let's say you had a chance to go to a special school for artists, but it was very far away from your home. OK, now your brother says, don't go away, I don't want you to leave me! How would you feel? I realize these are two very different things, but the feelings would be the same. You think this gal is trying to take you away, but where are you going? He will always be your brother. He may do things. go places and be with other people. You may not even like all these things. But to be fair, he has to be himself and experience things just like you do and will. If you say anything too harsh, he could just end up resenting you. Just tell him you miss him and when he has time, you'd like to hang out with him more. This gal can take some of his time away, but she can never replace you!
The best way to let your brother know that his girlfriend is taking him away from you is to face your brother and talk to him. It is true that their relationship has bonded them until he seems to forget you, but it is the right time to face him before it is too late.
Take a photograph with them ( brother and girlfriend) in same pic and check does she make your brother look good, if so, let them be and wish him all the best but if she's bad for your brother tell him to face the reality and get rid of that chick as fast as he can.
I had the same problem and so walked up to her and I punch the hell out of her and she walked up to my brother and said it was her or me and he said she is my sis and she's had my back ever since we were kids so if you do not like it than you can leave me but I will have her back forever so good bye bitch