NEW YORK (AP) - It's a family affair on NBC's ''Medium.'' Rosanna Arquette will guest star on sister Patricia's spooky show this spring, making her the third Arquette sibling to make a mark on the drama, NBC and show producer CBS Paramount Network announced Wednesday.

The episode, scheduled to air April 7, features the 48-year-old Arquette as ''a beautiful, calculating 'cougar' who enjoys the challenges of bedding younger men'' and pops up in the dreams of crime-fighting psychic Allison Dubois ''when her sexual escapades turn deadly,'' the networks said.

''I have to say I have been calling in a lot of favors and I'm really excited about this particular part they wrote for her because Rosanna is a very strong comedic and dramatic actress and this role requires both elements,'' said Patricia Arquette, 39, who portrays Dubois. ''Of course, every time one of my siblings comes on set, the crew realizes that I have yet another sibling nicer than me,'' she said, before praising her sister's ''great legs.''

The Arquette acting family also includes David Arquette, Richmond Arquette and Alexis Arquette. David recently directed two episodes of ''Medium,'' and Richmond guest starred on the show two years ago.

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