Anika Noni Rose and Taye Diggs

Sam's sister has arrived!

Corrine, played by Tony winner Anika Noni Rose, made a brief appearance in last week's episode when Sam (Taye Diggs) bailed her out of jail after she robbed a convenience store. First impressions aside, she's no common criminal, and despite what the promos lead you to believe, she's not just some drugged out, desperate woman. She's actually mentally ill.

Exclusive: Paul Adelstein to direct an episode of Private Practice

"She's bipolar," Rose tells TVGuide.com. "She doesn't know she's bipolar, she's been misdiagnosed many times. No one actually knows what her deal is. She's been drugged for so long and on the streets for so long, and also self-medicating to try to feel better, but it turns out that she is bipolar and they're trying to find a way to treat her."

Now that Sam has her in his care, he'll turn to Sheldon (Brian Benben) to try to figure out exactly what's wrong with her. "She's a little skeptical of the diagnosis because she's been diagnosed before and people have been wrong," she says. "She's been given medication before and it hasn't worked, so that's difficult, particularly for somebody who's fighting with a mental illness because the moment you start to feel better, you think you're better."

Through this process the siblings will finally be able to reconnect. "They were close at one point, and then estranged; that will be revealed more as the episodes progress," Diggs says. "They're trying to make up for lost time and reintroduce themselves to each other."

Get the exclusive details on the Grey's Anatomy-Private Practice crossover

Because of their new found connection, Sam will be in denial over his sister's illness. "With the people that are closest to you, you don't want to look at the truth," Diggs says. 'It's the situation where it can happen to everybody else but you. In this situation, something obviously is going on with his sister and he just doesn't want to believe it."

Though Addison (Kate Walsh) and Sam are broken up, she'll be there to support him through this trying time. "She lives right next door, so she sees first-hand what's happening and transpiring," Walsh says. "She's trying to be his friend and be there for him."

Private Practice airs Thursdays at 10/9c on ABC.