I'm not sure what your price range is, but I'd highly recommend Nordstroms for dress shopping. They have a wide selection and everything is nicely sorted by garment type. There's an absolutely gorgeous copper taffeta dress they have there (I'm not sure if that's a color you can wear either) that would work well with a shawl or cardigan for cooler weather.
Cocktail dresses in black are everywhere, I agree.
But it's so much more fun trying to find something different.
If Shelli Segal and Betsey Johnson are out of your price range (my personal faves for dresses), XOXO and other juniors brands carried at Dillards and Famous-Barr (I'm not sure if you have either of those where you are, but any mid-range department store should have them) often have cute stuff for semi-formal dances that are pretty versatile and very wedding-appropriate.
Can you tell I'm a shopping junkie?
Good luck finding a dress and congrats to your sister on her wedding.
PS Your PM box is full! Everybody wants to give you dress advice, I guess.