Britney's Custody Battle: It's On!

Kevin Federline is not kidding around. Britney Spears and her cousin/friend/assistant, Alli Sims, were hanging out at a party Saturday night, when Alli was suddenly served with legal papers concerning the custody battle between Brit and K-Fed. Alli was on her way out of the party in the Hollywood Hills (Brit was still inside) when she was served by Aaron Cohen of IMS-Security. "Alli was really shocked and covered her face," a witness told PEOPLE. "[She was] really distraught and said 'Oh my God.' "

Britney, who's said to be furious, left the party minutes later, and met up with Alli back at Brit's home. Kevin, who has just recently filed papers seeking primary custody of their sons, Sean Preston and Jayden James, is looking for Brit's best pal to take the stand and testify about the star's mothering skills. Us Weekly spoke to a source close to the custody case who says K-Fed's lawyers will use Alli to strengthen their case. They plan to "ask her about everything. What she feeds the kids, what's going in the bottles, how late they're staying up and about what Britney is like when she parties."

This is not going to be good for Britney -- at all. For more on Britney's custody battle, watch Daily Blabber TV.

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