You know what Capture the Flag is, right? Capture the Sister is remarkably similar, except instead of running away with a colorful piece of cloth mounted atop a pole, the name of the game is to kidnap a little sister, bring her back to your own neighborhood and stuff her into one of those weird Little Sister Holes that are peppered throughout Rapture.
It looks like both Splicers and Big Daddies will figure into the combat, although how they'll be mixed across teams isn't made clear. One Daddy per team, perhaps computer-controller protectors of the Little Sister? The video doesn't show them tangling with one another so I would expect some limitations to be imposed on their use.
Whatever the case, the inclusion of a CTF-style game mode is hardly surprising given the new focus on multiplayer support in BioShock 2, but does the world really need another minor variation on flag-capturing games? Seriously, I'm yawning here. Somebody give me a shout when 2K announces BioShock Bombing Run and then maybe we'll have something interesting to talk about.