Grey's Anatomy is expanding the Shepherd family tree.

The ABC hit is introducing Derek's (Patrick Dempsey) fourth sister, Kathleen, at some point during Grey's Anatomy Season 15, according to TV Line.

If you watch Grey's Anatomy online, you know that Kathleen has been referenced several times throughout the show's run, but we've never actually met her in the flesh.

What we do know based on previous episodes is that she's a psychiatrist. Whether that means an arc will bring her to the hospital, we have no idea.

But it's possible she could be stopping by to visit Amelia, (Caterina Scorsone) her sister who remains one of the best surgeons in Grey Sloan Memorial.

The TV Line report confirms that the hour Kathleen makes her grand entrance with an Amelia-centric one, so we should be in for a lot of scenes involving the siblings.

It's unclear whether Meredith (Ellen Pompeo) and Kathleen get on, but it would be ridiculous not to have these two characters share the screen at one point.

The standalone episode for Amelia was announced shortly after ABC extended the current season to 25 episodes.

"I knew for a long time that we were making 24," Grey's showrunner Krista Vernoff told press when the news was announced.

"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."

More Grey's Anatomy is always a good thing, right?

The veteran ABC hit is showing no signs of slowing down ... even after fifteen seasons.

While there has been a lot of chatter that the show could come to an end with its forthcoming sixteenth season, new ABC boss Karey Burke was optimistic about the show going on longer.

“The show was picked up for two seasons last season, so we have another season to go and I’m sure we’ll be planning for more,” she told Deadline at TCA earlier this week.

"We’re thrilled with Grey’s performance; I was visiting the set earlier this week. I believe the status is that we’re in conversations about continuing the show.”

Maybe the show will outlive us all!

What are your thoughts on the fourth sister being introduced?

Hit the comments below.

Grey's Anatomy airs Thursdays on ABC.