And apparently, the multiplayer follows the Civil War in Rapture.
Leveling up your charcter and gaining more plasmids to use deforms them into those cheery, completely sane people that wander Rapture by the time you visit of your own free will.
There isn't a singular Big Sister, there is a team of them, being used by Raptures new dictator, some sort of pshychologist, to maintain order, which is to say, keep the Splicers low, and herself high.
Though, there is a Big Sister, which probably means some sort of character development for it.
Apparently.
[video=6242031]
Here is a video. IF that doesn't work, here is a link:
http://au.gamespot.com/xbox360/action/bioshock2/video/6242031/bioshock-2-interview-gameplay-and-story-exclusive-interview?tag=videos;title;4