The Big Sisters are Little Sisters from the original Bioshock (aged by 10 years), that traveled to the shores to kidnap little girls to re-populate the little sisters in Rapture.
They are one of the main enemies in Bioshock 2 and can come in groups no larger than two, but most commonly one will appear to face you. They wield a long needle on their arm that is used for extracting ADAM. They are very quick and agile and a force to be reckoned with.
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