Little Sisters are young girls who have been genetically altered and mentally conditioned to reclaim ADAM from the corpses around Rapture. Little Sisters are almost always accompanied by a Big Daddy. They are almost completely immune to damage and have no offensive abilities. Attacking them, however, will incur the wrath of their protectors.
info found at <a href= http://bioshock.wikia.com/wiki/Little_Sister</a> Bioshock wiki