"Grey's Anatomy" Is Finally Introducing Derek's Fourth Sister And TBH, It's My Dream Casting

Kathleen Shepherd is coming to Seattle!

Let's go back in Grey's Anatomy history for a second (I'm talking Season 1) when Derek told Meredith that he had four sisters.

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Hmm...okay, so four Shepherd sisters. Let's do some counting.

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First, we met Nancy Shepherd in Season 3 when she arrived in Seattle to check in on Derek.

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Then, we met our beloved Amelia Shepherd in Season 7 after Derek refused to call her after he was shot.

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In Season 9, we met Lizzie Shepherd when she donated a nerve in order to fix Derek's hand after the plane crash.

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But, Derek's fourth sister has remained a Grey's mystery until right now!

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According to Deadline, Angel and The Gifted star Amy Acker has been cast as Kathleen Shepherd for an upcoming episode this season.

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Kathleen will make her long-awaited appearance in an episode that will center entirely on Amelia.

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Kathleen was only mentioned by name in a Season 9 episode when Meredith and Heather were looking for a nerve donor for Derek.

In case you're not totally caught up, Amelia has been dealing with fostering Betty, a teenage mother and addict, and trying to navigate her complicated relationship with Owen.

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While ABC is keeping things pretty secretive about what exactly brings Kathleen to Grey-Sloan, I'm sure it's bound to be an emotional Shepherd sister reunion.

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Also, besides Amelia, this is the first sister we're seeing since Derek died in Season 11, which could possibly lead to a confrontation between Meredith and Kathleen because she wasn't with Derek when he died.

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So, personally, I'll be stocking up on tissues and eagerly awaiting Kathleen's arrival at Grey-Sloan Memorial Hospital.

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