That us a very mean thing for a sister to do. I'm sorry you have to deal with the situation. I'm sure you feel hurt and betrayed. I'm sorry. You need to keep busy with your friends. And stay clear of your sister. Find activities that keep you involved.
Does your sister hate you? Why would she do that? Blood is blood, you guys should stick together against everything, she'll be sorry someday for this. It'll be the day he dumps her for her best friend. I'm not sure what to tell you in the meantime, what a betrayal! After he dumps her, and he will, you guys gotta talk and make sure this doesn't happen again. Is she immature or something?
Find someone better then that jerk that would even go out with your sister and if your sister loves you she would ask you about it and how you feel and you should tell her how you feel and go out and find someone better then him. :)
Same thing happened to me, and now it's still happening to me.talk to your sister or just live your life but don't act like your showing off just let yourself be happy and try to ignore them. i did both