Most Helpful Girl
Your brother is an asshole for doing this. As for her be thankful you are rid of this bitch.
Most Helpful Guy
What do you do?
Just ignore her, forget about her. If she tries to contact you by any means then just block her completely.
Both your brother and your girlfriend are equally responsible, your brother because what he did was inappropriate and wrong despite knowing she is your girlfriend. Your girlfriend because she justified his moves by sleeping with him.
You deserve better, you don't need her in your life.