Levi Johnston's Sister Appearing Nude in Playboy

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Levi Johnston isn't the only model in the family.

ET has learned that Levi's younger sister Mercede will be featured in a nude spread for Playboy's September issue.

The spread is accompanied by a candid interview where the 19-year-old discusses how she really feels about her brother's relationship with Bristol Palin.

"Levi was whipped like Todd, because Bristol is very manipulative," Mercede said. "She'd use sex to get her way on many things."

Concerning Bristol's pregnancy, Mercede alleged that it wasn't an accident, saying, "She and Levi planned it. They were trying to conceive for months."

Read the entire interview when the September issue of Playboy hits newsstands Friday, August 12.