When it comes to casting for her new NBC drama Barely Legal, Hilary Duff is keeping it all in the family!

The 21-year-old, who will play a legal wiz who passed the bar exam at 18, has demanded that sister Haylie Duff, 23, be written into the show.

"It’s a challenge," a source inside the show tells OK!. "Haylie will play the big sister who visits from time to time."

Meanwhile, Hilary was spotted preparing for her return to TV at an L.A. gym on Jan. 14.

"She’s determined to get her body back," the source tells OK!.

And don’t think the star has her heart set on just her own show!

"[NBC has] great shows to have arcs on," Duff said recently. "I would love to do something on The Office. 30 Rock is great, very funny. Many people have done lots of guest spots on it this season."

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