Medusa's unnamed sister (Stefanie von Pfetten) was a minor villainess from "The Rocks," episode 2.06 of Special Unit 2 (airdate November 7, 2001).
She is an evil gorgon and the sister of the episode's main villainess, Medusa. Both she and Medusa regularly frequented nightclubs, where they would seduce men and later turn them into stone statues.
The villainesses encountered Nick while he was working undercover to search for the gorgons. The sister enticed Nick by offering to participate in a threesome with her and Medusa, but once they exited the elevator, the duo revealed themselves as gorgons and went after Nick. At that moment, Kate appeared and shot Medusa's sister, killing her by petrification, while Medusa escaped.