You know I'm obligated to answer any question pertaining to clothes/fashion/fabric care/whatever, so here goes...
1. Ask any of your friends who are the same size as you if they have anything you could wear. I borrowed my last formal dress from my best friend.
2. If that doesn't turn up satisfactory results, borrow the dress from your sister IF you like it as well as yours. Wearing the same dress again isn't really a huge no-no (hey, I've done it...er...once...or twice...or...a lot
), though...if you like yours and think it looks better, then wear it.
Have fun!