Amy Acker (The Gifted, Person of Interest) has been cast in a guest starring role as Derek Shepherd's fourth sister, Kathleen, per The Hollywood Reporter. Of Derek's four sisters, only three have appeared on screen (Embeth Davidtz as Nancy, Neve Campbell as Lizzie and Caterina Scorsone as Amelia). Kathleen will appear in an Amelia-centric episode later this season per Deadline.
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