SISTER ACT

‘Grey’s’ star Ellen Pompeo directs her on-screen sib

AFTER 13 seasons on the air, it’s hard for “Grey’s

Anatomy” to experience a “first” anymore.

But Thursday’s episode managed to do so with the directorial debut of series star Ellen Pompeo, who’s played Dr. Meredith Grey on the popular ABC drama since its 2005 premiere.

The hour she directs centers on Meredith’s half-sister, Dr. Maggie Pierce (Kelly McCreary) and her mother Diane (LaTanya Richardson Jackson), who came to Grey Sloan Memorial for a mastectomy in last week’s show. After the surgery revealed that her inflammatory breast cancer is more severe than previously thought, Thursday’s episode (8 p.m.) sees Denise’s health deteriorate — and the doctors at odds over how to treat her.

“[Ellen] brought so much to this particular script,” McCreary, 35, tells The Post. “She’s got the depth of knowledge of these characters after 13 seasons on the show. And she has a personal connection to this story being that she lost her mom herself.

“It enabled her to find some really beautiful moments both in our performances and draw even more out of what we were giving her,” she says. “It was amazing synchronization.”

McCreary’s character — who joined “Grey’s Anatomy” in 2014 as Grey Sloan Memorial Hospital’s head of cardiothoracic surgery — initially got off to a rocky start with her half-sister Meredith (Maggie is the love child of Ellis Grey and Richard Webber and was put up for adoption). Over the seasons, the two have become friendly, eventually

even moving in together. But Diane’s cancer will again strain the sisters’ relationship as they disagree about how to tackle the severity of the disease.

“What’s up for debate is how aggressively to go at the treatment,” McCreary says. “Often these treatments and their side effects can be more painful than the disease itself. It’s a question of what is the most humane thing to do — what does Diane want to do versus what does Maggie want to do versus what do the experts that they’re all surrounded by think is wise?

“[Maggie and Meredith] are in conflict in the beginning but that will change as the situation with Diane’s health changes throughout the episode.,”

Despite directing, Pompeo appears in the episode as much as she normally would. In the hour, viewers will also get to meet Maggie’s dad, Bill (Richard Lawson), who recently split from her mom. While McCreary can’t reveal what brings him to Seattle and to Grey Sloan Memorial, there will be a run-in between her adoptive and biological father, Dr. Webber (James Pickens Jr.).

“It’s incredibly touching, the moment between Maggie’s dad and Dr. Webber,” she says. “There’s so much going on in that moment, there’s a tension that neither dad expects to experience in that moment because of the events that led up to it.”

“Grey’s Anatomy” 8 p.m. Thursday on ABC

“She has a personal connection to this story being that she lost her mom herself.” — “Grey’s Anatomy” co-star Kelly McCreary (Maggie) on series star Ellen Pompeo