Grey’s Anatomy is going to introduce Derek Shepherd’s fourth sister

It looks like Grey's Anatomy is set to introduce Dr. Derek Shepherd's fourth sister - finally. 

Kathleen Shepherd has been mentioned before on the show, but has never actually made an appearance at Grey Sloan Memorial (or in Seattle, really). The character, which has not yet been cast, will appear in an Amelia-centric episode due to air later this season, according to TV Line.  Like her siblings, Kathleen - or Kate - is a doctor. Her speciality is in psychiatry. Besides Amelia, Derek's two other sisters - Liz and Nancy - have both made appearances on the show before. [caption id="attachment_449269" align="alignnone" width="640"] Nancy Shepherd[/caption] Neve Campbell played Liz during an appearance in season 9, while Embeth Davidtz played Nancy in season 3. 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_449270" align="alignnone" width="466"] Liz 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." There isn't too much information about the special episode out there yet, beyond the fact that it apparently won't take place at Grey Sloan Memorial.