According, an “amazonian” type of Big Daddy. Supposedly sleeker and taller, while wearing the same sort of outfit, except more feminine obviously. According to Kotaku’s sources, Big Sister’s will be in Bioshock 2.
There was a picture shown on the official teaser website, a week or so ago, that showed a doll resembling Big Daddy. But there were few differences, I’m guessing that the doll was actually Big Sister. The description found on the official teaser website describes the “Big Sisters” as tall and very fast. GDC is coming up, and there’s a chance that this will be talked about. Still, it sounds pretty interesting to me. A fast version of the Big Daddy’s, that sounds fun, lol.