Not to try to make matters worse or anything, but if a guy can get sucked fro ma girl more then likely he will..
I suggest showing up at your girlfriends house and confronting her face-to-face with the situation. Definitely talk to your brother though, no matter how pissed you may be at him, it is the only way to figure out all of what happened. Honesty is a very big key factor in life.
You should not go back out with her, that's for certain. And try your best not to pummel your little brother to bad.
Just a side note, that if you do not want to answer you don't have to. Have you and your girlfriend "did the do"?
EDIT: Seems you already got it handled, sorry for posting.