CVG has compiled what we know so far about BioShock 2's storyline, based on the small amounts of information revealed on 2K Games' official website. They also throw in a bit of speculation along the way:
The first update on Something in the Sea shows a hand-made child's toy "of most singular design" discovered on a nearby beach, said to resemble the peculiar being.
There was also a discovery of strange, small footprints on the neighbouring sand alongside the item. "The metallic soles do not resemble any civilian or military footgear," claims the man on the scene, pointing out the assailant's "unusually long stride."
So what exactly is a "Big Sister"? So far there's no official answer but one possibility lies with the fate of Bridgette Tenenbaum, the genetic scientist who created Adam and helped you free the Little Sisters from Rapture. Tenenbaum's fate is unclear at the end of the first BioShock, but it's likely she was left behind as the only intelligent being alone in the city.
Now this is going out on a limb, but we reckon if we were left alone in an underwater dystopia, full of crazy murderers and mutants for seven years... we'd go a bit mental. And it's possible that Tenenbaum has too. In her insanity has the good doctor suited up and gone on a quest to snatch back her children? That's where our money is, anyway.