Grey’s Anatomy‘s forthcoming Amelia-centric episode has officially morphed into a full-fledged event. TVLine has learned exclusively that the Caterina Scorsone-driven hour will at long last introduce late Derek Shepherd’s oft-mentioned-but-never-seen fourth sister, Kathleen.

An ABC spokesperson confirmed that casting for the pivotal role is underway, but the rep declined to provide further details. Little is known about the mysterious Kathleen (aka “Kate,” but it was previously established that she — like her other siblings — is a doctor (her speciality being psychiatry).

Of Derek’s other three sisters, only Scorsone’s Amelia remains on the show. The roles of Liz Shepherd and Nancy Shepherd were previously played by Neve Campbell and Embeth Davitz, respectively.

“I’ve been doing the show for a long time and just hearing what [the episode is] about fills me with nervous excitement,” Scorsone previously enthused to TVLine of the Amelia-focused hour, which will be written by Julie Wong and directed by Bill D’Elia. “It’s not just, ‘Let’s follow Amelia to the neuro lab.’ It’s definitely a little standalone. It’s almost like a little movie on its own.”