Rosanna Arquette, <EM>Medium</EM> Rosanna Arquette, Medium

The last time they performed together was as kids in their parents' living room, but Patricia and Rosanna Arquette will finally reprise their act on the April 7 episode of Medium (Monday at 10 pm/ET, NBC) — albeit their roles are slightly different this time.

Rosanna guest-stars as a femme fatale in a murder case Patricia's visionary Allison DuBois character is investigating. "This episode is about this black-widow woman who's seducing men and killing them," reveals Patricia, who has been trying to convince her older sister to appear on Medium for four years. "It's fun because our killers aren't usually women."

Says Rosanna, "When [executive producer] Glenn Caron called, I was like, 'Ooh, I'd like to be a killer! A lady killer.'" The feeling was mutual. Caron has been a fan of Rosanna's since he first saw her turn as a love-struck teenager in the 1983 flick Baby It's You. "One of the nice things about being in business with Patricia Arquette is you get access to all of these other Arquettes," jokes Caron, who previously hired brothers David (to direct) and Richmond (to act) for Medium. "I can buy them, trade them, absolutely. It's like having your own little repertory!"

See full episodes of Medium in our Online Video Guide.

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