‘Grey’s Anatomy’ Set to Introduce Derek Shepherd’s Fourth Sister

Grey’s Anatomy will finally introduce the last of Derek Shepherd’s sisters fans have not met yet.

The long-running ABC medical drama will show Derek’s sister, Kathleen, for the first time in the upcoming Amelia-dedicated episode, set to air later in Season 15.

According to TVLine, casting is currently underway for the key role — Kathleen has been mentioned often on the series since the early seasons. Not much is known about Kate, though she has been revealed to be a doctor, specifically a psychiatrist.

With the introduction of Kathleen, fans will finally see the faces of all of the members of Derek’s family. The show previously introduced Liz Shepherd and Nancy Shepherd, played by Neve Campbell and Embeth Davitz in previous guest appearances.

Amelia (Caterina Scorsone) was first introduced on Grey’s Anatomy spinoff series Private Practice. Scorsone was a series regular for the past three seasons of the series. She later joined Grey’s Anatomy full time in Season 11 and has stayed on the show ever since.

Grey’s Anatomy previously announced Amelia’s standalone episode — set to be written by Julie Wong and directed by Bill D’Elia — following the announcement Season 15 had been upgraded to 25 episodes.

“I knew for a long time that we were making 24,” Grey’s showrunner Krista Vernoff told press when the news first broke. “We’ve been making 24 episodes a year for many years now. ABC asked about [doing one more] and I spent a week thinking about whether or not we could accomplish it. I [ultimately] said yes because I like to be a team player.”

The extra episode gave writers the opportunity to dedicate the hour to Amelia.

“I had an extra standalone episode idea that I wanted to do this season,” she added. “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.”

Not much is known of the plot of the upcoming episode, other than it will take place outside the walls of Grey Sloan Memorial.

“I’ve been doing the show for a long time and just hearing what it’s about fills me with nervous excitement,” the actress teased, revealing to the outlet that the episode will feel like its own “mini movie’ not taking place at the hospital.

“It’s not just, ‘Let’s follow Amelia to the neuro lab,” Scorsone joked. “It’s definitely a little standalone. It’s almost like a little movie on its own.”

Grey’s Anatomy airs Thursdays at 8 p.m. ET on ABC.