Tough call, man. I know how you feel, but I would show up there and greet her/ask her how she's doing (common courtesy) but no more than that. You gotta show yourself that you're strong enough to deal with your issues. Being in the same room with her and actually focusing attention to something else (your sis' bday) is going to be an important part of your healing phase. Yeah, it's too early, but man life is tough. I believe your sis will not feel the same if you're not there especially if you are close.
I can only imagine how much you were in love with this girl. But it's time to move on NOT move away!
Let us know how it went...
Good luck and try to talk to other girls at that party. After all that's what parties are for.