The Shepherd family is finally complete.

Grey's Anatomy have officially cast Derek Shepherd's fourth sister. Last week, it emerged that the long-running medical drama would be introducing another one of Derek's siblings. Now, TVLine report that Amy Acker - who has previously starred in Suits, Angel and Person of Interest - will be playing Kathleen (or Kate) Shepherd. [caption id="attachment_450166" align="alignnone" width="466"] Amy Acker[/caption] She's been mentioned before on the show, but has never actually made an appearance at Grey Sloan Memorial (or in Seattle, really). Or, at least, not that fans have seen. Like her siblings, Kathleen - or Kate - is a doctor. Her speciality is in psychiatry. [caption id="attachment_449270" align="alignnone" width="466"] Liz Shepherd[/caption] Derek's two other sisters - Liz and Nancy - have both made appearances on the show before. Neve Campbell played Liz during an appearance in season 9, while Embeth Davidtz played Nancy in season 3. Amelia Shepherd is the only one of his sisters that remains on the show. The Amelia-focused episode was announced earlier this year, when ABC revealed that the were ordering an additional three episodes for season 15. [caption id="attachment_450165" align="alignnone" width="640"] Nancy Shepherd[/caption] This brings the total number of episodes for the season to 25. Showrunner Krista Vernoff previously told TVLine:

"I had an extra standalone episode idea that I wanted to do this season. So it doesn’t really lengthen our season too much. "It allows us to tell [this Amelia-centric] story that we might not otherwise have gotten to tell this season."

Caterina Scorsone said that it was "almost like a little move of its own", explaining that it "fills me with nervous excitement."