the big sister is on the cover. it is a little sister that was not saved in the first game and is now a more grown up version of genetically engineered jailbait. you will be playing as the first big daddy ever made and somehow you will get the ability to switch around what kind of main weapons you use (so you can use conventional bullets as well as the drill thingy) and you can even pump some gene tonics into yourself.
it sounds like the big sister will be a boss that you fight multiple times.