For a fun sisters-only night out, I would suggest bluezoo. They easily have the best bar on property, what with their very knowledgeable staff, interesting menu, and sophisticated and sexy ambiance. The restaurant proper is a bit more subdued, but still fun and adult. They do a great job with their seafood offerings, and one can easily make the argument that it's the best seafood restaurant within the Disney bubble. Really, any of the Swan/Dolphin signatures would be a good choice, but bluezoo would be my pick of the three for you two (Il Mulino being a respectable second; I love Shula's, but you really have to be in the mood for steak and football decor).
If you're looking for something a bit more casual, I would recommend Sanaa for a restaurant, and the Cabana Bar for a lounge. The former is my "bang for your buck" pick, since it's a beautiful restaurant with delicious food at good-for-Disney prices. It's really close to signature, but without any fussiness. The latter is the partially outdoor pool bar at the Dolphin. They have a limited, but very good, food menu, and a fantastic drink menu. Go and get a Scorpion Bowl of Dark and Stormy. It will be the only drink you need for the entire night, and it's downright affordable for the amount you get.
I don't think the Swan/Dolphin options get nearly enough play on here, in general, but in your case and situation, I can't think of a better option.
