Rosanna Arquette portrays a cougar on Medium alongside younger sis, Patricia.

When considering the Arquette family, we can't help but recall a classic utterance from The Larry Sanders Show.

The subject of the acting Baldwin brothers comes up, and Artie, played by Rip Torn, says something along the lines of, "I don't like those Baldwins -- there's too many of them, just like those goddam Carradines."

If The Larry Sanders Show still were on the air, Artie undoubtedly would be creeped out by "too many Arquettes" on Medium tomorrow night.

Medium, which airs on CTV and NBC, is creepy enough on its own from a conceptual standpoint. It stars Patricia Arquette as Allison DuBois, who uses her psychic abilities to solve crimes.

Tomorrow Patricia's celebrated older sister Rosanna Arquette guest stars on Medium. It's the first time they've worked together on a fictional project.

"Poor Rosanna, her first day (on the set) was a love scene," Patricia said in a recent conference call.

"Two days in a row of love scenes," Rosanna corrected.

"I kept saying, 'Am I going to work with my sister?' " Patricia added.

"I thought she was playing a joke on me," Rosanna said.

The episode is titled Lady Killer. Rosanna plays a cougar -- not the animal, but the other kind of cougar -- whose sexual conquests often result in young men dying.

"I've never been one to go out with younger men but it did spark an idea in me," Rosanna said. "Like, wow, this is not so bad."

Rosanna and Patricia didn't grow up as contemporaries. Rosanna is 48. Patricia turns 40 on Thursday. That eight-year gap is nothing now, but when they were kids, obviously, they were in different worlds.

"I babysat a lot but I left home early so I missed some of their years," said Rosanna, referring not only to Patricia, but also to siblings Richmond (44), Alexis (born Robert, 38) and David (36), all of whom are in showbiz. Artie just keeled over, by the way.

"As we got older we became closer," Rosanna continued. "I mean, they were my babies, they were little kids."

Patricia has childhood memories of Rosanna teaching her to do "the bump." It's a dance, and if you don't remember it, don't ask.

"We would listen to her records, like the Jackson Five," Patricia said. "I always thought my sister was really cool. I'd get her old makeup and I got the best hand-me-downs."

Rosanna remembered that when she was on the set of the movie Desperately Seeking Susan in the mid-1980s, Patricia showed up on the doorstep, sporting a punk hairdo, shaved bald on one side.

"Rosanna was the first person I ever told I wanted to be an actress, which shows how safe I felt with her," Patricia said. "And she said, 'Really? Okay, there's always room for another rose in the garden.' She's the one who paid for my acting classes."

Desperately Seeking Susan, as you'll recall, co-starred a young singer named Madonna.

"It happened during the making of that movie," said Rosanna, referring to Madonna's exponential rise from stardom to mega-stardom. "They started re-writing the script for that, to tailor-make it for her.

"And here she is, still at it, almost 50 and rocking on."

Madonna at almost 50, eh?

Speaking of cougars ...