This news, while awesome, isn't a huge surprise. After all, Ellis was looped back into the Grey's storyline in the Season 10 finale, when new head of cardio Maggie Pierce (Kelly McCreary) revealed that Ellis is her mother — seemingly unaware that the man who she made her confidant, Richard (James Pickens Jr.), is her father. And Mer certainly doesn't know that she has another half-sister working at the hospital.
Ellis, who only really bonded with Meredith after she and her career had been sidelined by Alzheimer's, passed away during Season 3. She did have a brief cameo, however, in the alternate-reality episode of Season 8.
Kate Burton also played a younger version of her character in various flashbacks throughout the show, but in an Ellis-centric Season 6 episode, however, the younger Ellis was played by American Horror Story star Sarah Paulson.
Even as recently last month, Shonda wasn't sure which Ellis we'd be seeing, telling The Hollywood Reporter, "It was the exact same time period, maybe months apart, somehow played by two different actresses. So I don't know if it's going to be Sarah Paulson or Kate Burton, we'll figure it out."
She also says that Ellis's secret lovechild is not an eleventh-hour plot twist — the Grey's writers have been establishing the framework for the reveal for years. "It's literally always been layered into the storyline," she explains to TV Guide. "We can literally go back and pull out moments of the show, and I'm excited to do so and point out where it was talked about. It's not going to be so hard to do."
Looks like we're gonna see that backstory sooner rather than later!
Sources: TV Fanatic, TV Guide, The Hollywood Reporter