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According to TVLine Neve Campbell, of Party of Five fame, will play Derek's (Patrick Dempsey) sister for an upcoming multi-episode arc.
There's no word yet on which sister she will play, but we know he has four. We've already met Amelia and Nancy. He also has a sis named Kathleen, who is a psychiatrist, and a fourth sister we know little about.
Right now, we'd tentatively guess Neve is playing Kathleen, because we'd bet Derek — and everyone else at Seattle Grace-Mercy West! — will be in the need of some good psychiatric care in the wake of the crash, whether he wants to admit it or not.
Catch the next new episode of Grey’s Anatomy on Thursday, October 18 at 9 p.m. ET/PT onABC.