Patrick Dempsey, Caterina Scorsone and A.J. Langer

The bad news: Erica, who had a child with Private Practice's Cooper, has a terminal brain tumor. The good news: Amelia Shepherd and her big brother Derek will reunite to try to save her.

Amelia (Caterina Scorsone) will travel to Grey's Anatomy's Seattle Grace during a special crossover episode, airing Thursday at 9/8c, in which she'll convince Derek (Patrick Dempsey) to lend a hand in saving Erica (A.J. Langer) — something that has thus far seemed impossible.

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"We're coming at the case from such a place of hopelessness in terms of Erica's prognosis, that it can only go up from here," Scorsone tells TVGuide.com. "There's pressure for Amelia to do the best job possible, but Erica is going to die, that's the hard reality that everyone's been dealing with for this whole story arc, so even the fact that there's a possibility that she could live is something better than what we had thought an episode [ago]."

Erica has long resisted the idea of treatment, fearing it would limit her time with her son Mason (Griffin Gluck). And it isn't foolproof: The difficult procedure requires the surgeons to remove the tumor in less than 90 seconds. "It really is a dangerous procedure that they're attempting to do, so Erica's quite shaken by how immediate the option is," Scorsone says. "She has to decide within a very short amount of time, and if it doesn't go well, that's going to be the last time she sees her son, so that's a huge decision for anyone."

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"Once she's there, and she's presented with what the procedure would actually entail and how risky it is, and it almost sounds impossible, she has even more reservations about it," adds Paul Adelstein. "The biggest challenge of all to Erica is that she still hasn't told Mason."

Erica's case will also mark the first time the Shepherd siblings have been together since Amelia's recent trip to rehab. Though Derek was missing from Amelia's big intervention earlier this season on Private, her falling off the wagon will be addressed during the crossover. "He's the one who brings it up," Scorsone says. "Families are complicated and there's a lot of history between the two of them. They've shared good times and bad, but ultimately, they have a bit of a confrontation about it, but it's resolved in an interesting and positive way."

Don't expect Amelia, who ventured to Seattle Grace last season, to make some of the same mistakes again — like sleeping with Mark Sloan (Eric Dane). "There's no time for dilly-dallying," she jokes. "Amelia has lives to save!"

The Grey's Anatomy-Private Practice crossover begins at 9/8c on ABC.