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Kevin Federline claims his wife, Victoria Prince , remains in touch with Britney Spears ' family ... and his tell-all memoir features a couple messages he says Victoria got from Britney's sister, Jamie Lynn Spears .

In excerpts from "You Thought You Knew," obtained by UsWeekly -- Kevin claims Jamie Lynn messaged his wife about Britney "lashing out" on social media at the two boys she shares with Kevin ... Sean Preston and Jayden James . Getty

K-Fed claims Jamie Lynn wrote ... "Honestly, this makes me want to cry because I feel for those boys so much. I’m her little sister by 10 years, and I can’t imagine what her actual children must feel like at times, but I am familiar with the feeling of being a dingy trying to survive in a wave caused

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