The only thing I can say is to hold your head up high. Your sister is crashing the party, not you and you are not a criminal to be with her when she arrives - she's your sister! Let your stepmom say what she want to say, just do whatever you can to let it wash over you - this isn't your problem, it's hers (and your sisters) and you didn't do anything wrong. Is your father supportive of you?

Good luck tomorrow... and try to remember, things tend to be a lot worse in our heads than they actually turn out to be. I hope you enjoy some of it!