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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 greater natural affinity for the powers that ADAM creates."