Meredith (Ellen Pompeo) and Riggs' (Martin Henderson) secret hookup may shock a lot of their colleagues when "Grey's Anatomy" returns for season 13.

The unlikely pairing happened when Mer and Riggs had a confrontation after Mer had a hard day at work. The two were arguing inside her car, but it eventually turned into a steamy physical encounter afterwards. But while Mer made it clear that the incident was just a casual encounter for her, it appeared like Riggs already had feelings her by the way that he looked at her during her sister-in-law Amelia (Caterina Scorsone) and Owen's (Kevin McKidd) wedding.

But after McKidd revealed in an interview with Entertainment Weekly that his character Owen may not be thrilled to learn that Mer is hooking up with his missing sister's ex-boyfriend, Scorsone also revealed in a separate interview with the same publication that Amelia will also be caught in the middle of the relationship drama, since Mer's half-sister Maggie (Kelly McCreary) is also attracted to Riggs and believes that he feels the same way for her.

"Right now, Amelia doesn't know about either of their situations with Riggs, so that will be something really fun to explore next year – how she's going to find out about Meredith and how she's going to find out about Maggie, and how they're going to deal with it," Scorsone stated.

But the actress is hoping that the sisters will not let a guy come between them, after everything that they have been through in the past.

"I don't know that they'll necessarily be at odds. Sisters before misters. It's possible that they'll both try and accommodate one another. I'm sure Amelia will do whatever she can do to ease anyone's pain in that family," she added.

ABC will air the premiere episode of "Grey's Anatomy" season 13 on Thursday, Sept. 22.