If you thought tensions were already high between Meredith (Ellen Pompeo) and Amelia (Caterina Scorsone), just wait until you see how the sisters will react when Penny (Samantha Sloyan) makes her Grey Sloan debut during episode 6 of “Grey’s Anatomy.” According to a sneak-peek video of the upcoming Season 12 installment, titled “The Me Nobody Knows,” Amelia will lash out at Meredith for having Penny, the woman partially responsible for Derek’s (Patrick Dempsey) death, on her service.

“It’s not enough to have her here in the same building, you invited her on your service too?” Amelia angrily says to her sister. But little does Amelia know that it was Bailey (Chandra Wilson) who placed Callie’s (Sara Ramirez) girlfriend under Meredith’s watch.

“Dr. Blake is on your service,” the Chief of Surgery says to Meredith in the trailer for episode 6. And from the scorned look on Meredith’s face, it’s safe to say that she's initially not too pleased to babysit the woman who let Derek’s life slip through her fingers. However, Meredith will eventually understand the madness behind Bailey’s method.

“I am trying to rise above and you should too,” Meredith says to Amelia, trying to explain why she’s embracing Penny. But Mer’s sister is too blinded by anger to be understanding.

“You should have ended it that night,” she barks back at Meredith, insinuating that Meredith should have forced Penny to relocate to another hospital during their dinner party from hell.

In addition to seeing Amelia bump heads with her sister in the sneak-peek video, viewers are also given a glimpse at how Amelia will treat the unwanted doctor in episode 6. While Penny and Meredith break down a case for Amelia, Stephanie (Jerrika Hinton) asks if the man that’s having his head examined is the pastor who gave his congregation a “special blessing.”

“I bet you the pews will be full on Sunday,” Penny tries to joke with her fellow surgeons only to have the room fall silent.

“Grey’s Anatomy” airs Thursday nights at 8 p.m. EDT on ABC.