'Grey's Anatomy' Casts Amy Acker as Derek's Fourth Sister

Amy Acker
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The ABC medical drama will finally introduce the mysterious fourth Shepherd sister in season 15.

The mysterious fourth Shepherd sister is officially coming to town.

Grey's Anatomy has cast Amy Acker as Kathleen Shepherd, the last of the late Derek Shepherd's siblings, finally introducing the final piece of the Shepherd puzzle in the current 15th season, ET has learned.

Acker, who currently stars on Fox's The Gifted, will appear in the Amelia-centric episode, which is slated to air later this season. She is currently slated for one episode.

Acker's casting is significant as it rounds out the Shepherd family in the Grey's universe. Kathleen, who Meredith refers to as Kate, has often been mentioned but never seen. Like her family members, she is a doctor with a specialty in psychiatry.

Though little is known about Kathleen, it's been established that she is married with kids. In a season nine episode, Meredith credited her for being a good mother. In another episode, Meredith asked Derek's sisters to be contacted when it was revealed he needed a nerve donation. 

Neve Campbell and Embeth Davidtz played Derek's other sisters, Liz Shepherd and Nancy Shepherd, in seasons nine and three, respectively. Caterina Scorsone's Amelia Shepherd remains on the show.

Grey's will be celebrating a milestone in a few short weeks on Feb. 28, when it surpasses ER as the longest-running primetime medical drama, and it doesn't appear the show will be slowing down any time soon, with new ABC Entertainment President Karey Burke hinting that it will more than likely be returning for a 16th season

Grey's Anatomy airs Thursdays at 8 p.m. ET/PT on ABC.