Britney Spears' cousin and former assistant has reportedly joined forces with the pop star's estranged husband and mum in a bid to help him gain custody of their young sons. Alli Sims has teamed up with Kevin Federline and Lynne Spears after falling out with the Toxic singer, according to celebrity magazine OK!. The publication claims rapper/dancer Federline is seeking sole custody of his sons Sean Preston and Jayden James and Sims is helping him put together a case against her cousin. A source tells OK!, "Alli gave specific instances where Brit couldn't handle having the kids. "Kevin is worried about Sean and Jayden's safety. He had heard bits and pieces of what was going on from Brit's mother, Lynne, and from the bodyguards but has been trying to focus on getting along with her and didn't want to believe she had completely lost it."