'Grey's Anatomy' Preview: Webber's 10,000th Surgery Gets Complicated (Video)

Grey's Anatomy Gilpin Vardalos - H 2012

Grey's Anatomy Gilpin Vardalos - H 2012

On Thursday's Grey's Anatomy, a new set of sisters check in to Seattle Grace when former Frasier star Peri Gilpin and My Big Fat Greek Wedding's Nia Vardalos bring their sibling rivalry -- and bickering -- to Weber's O.R. for a liver transplant.

Aside from being a big case, Marcy (Gilpin) and Karen's (Vardalos) surgery also serves as a milestone in the Chief's (James Pickens Jr.) career as it becomes his 10,000th surgery. But Gilpin tells The Hollywood Reporter that the sisters complicated relationship create a less-than-ideal situation for Webber.

"Webber is very proud of this being his 10,000th surgery, and he's ready to go and very surprised at the attitude of both women going in," she says. "They're not in complete peace going into t he situation. I think he was probably hoping for something a little easier."

Gilpin notes that she and Varaldos wanted to present the sisters in a way where there's "no bad guy" despite the fact that Marcy certainly comes off in a less-than-loving fashion.

"It's just like every other sister relationship -- you always want what the other one has," she says.

"Sometimes when you're with other people -- friends or maybe your husband or wife -- you act brave but with your sister, you can be a little more honest," she adds. "Sometimes you have to let them bicker, it's like a relief valve and you have to let people fight it out a little bit."

Check out a clip from Thursday's episode, "Hope for the Hopeless," below. Grey's Anatomy airs Thursdays at 9 p.m. on ABC.

Email: Lesley.Goldberg@thr.com; Twitter: @Snoodit