Just when you thought Kevin Federline had faded into oblivion, his brother Christopher has apparently dropped a huge bombshell -- he says that Britney Spears cheated with him and that he fathered her first child, Sean Preston.

According to the National Enquirer, Chris has filed court papers in Tampa, Florida making the shocking allegations. The gossip mag says that Chris is looking to get Brit-Brit off his back, claiming that she's been blackmailing him about the affair. He also accuses Britney of stealing his credit card and running up a huge tab.

The legal filing seeks a temporary restraining order. No word on whether Christopher plans to pursue further legal action.