The singer's younger sister Jamie Lynn told Hello! that Federline has been accompanying Britney on tour to help take care of their two children.
She said: "Kevin has been great, as always. He and Britney have become best friends. She knows how much he cares about the boys and he knows the same is true of her."
Another source said: "Kev also gets on great with mum and dad, so it's one big happy family pulling together.
"Everyone just wants to make sure that this tour is successful for Britney and that she gets the comeback she deserves."
Last month, it was claimed that Britney had confronted Federline's girlfriend Victoria Prince about spending too much time with her children.
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