I don't want to go to my girlfriends sisters volleyball game
Is there a reason you don't want to go?
I don't want to hang around with people that I'm not that close with.
It's understandable that you might feel uncomfortable, however you should take a chance getting to know people. At the very least, it would be good of you to support your girlfriend's sister.
She should understand and respect my decision since she invited me.
It's possible that she invited you so that you can get to know the family better. Attending the game will show that you're willing to be supportive.