The Big Sisters of BioShock 2 were created when doctor Gil Alexander noticed that little sisters were not being protected to the full extent, and decided they needed a last defense.
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The Big Sisters are post-pubescent Little Sisters that have become unstable in Rapture's environment. Eight years after the events of BioShock, they are charged with maintaining the ecological balance in Rapture. Their extreme level of powers, far beyond normal plasmid users, can be attributed to their bodies adapting the ADAM created in their bodies as children while they were still developing. Thus they have a much more natural affinity for the powers that ADAM creates. They are extremely fast and agile, making them even deadlier opponents than the ponderous Big Daddies.The only game Big Sister are featured in is Bioshock 2.