The sexy redhead accused of spying for Mother Russia was on the feds' radar a decade ago -- thanks to her younger sibling, the N.Y. Post reported.

Slavic sweetie Katya Kuschenko, the sister of accused party-girl spy Anna Chapman, dated and has remained in close touch with a young man who's the son of an American intelligence worker and a US diplomat, the boyfriend's brother said.

Ten years ago, Kuschenko, then in her teens, dated 15-year-old Nathan Joseff, whose mother, Lena, was deputy chief of the US Embassy in Zimbabwe, according to Nathan's half-brother, Matthew Joseff.

At that time, Chapman's and Kuschenko's dad was the Russian ambassador to Zimbabwe, said Matthew, 40, a former NYPD auxiliary cop who is now an independent running for Congress in Connecticut.

Matthew said he and Nathan "have different fathers, but both our fathers were in the intelligence community. My mom, the deputy chief, was required by her job to disclose [to US officials] that her son was dating a diplomat's daughter, a foreign national," according to the N.Y. Post.

"She told the embassy," Matthew Joseff said.

Matthew learned of that fact only yesterday, when Lena Joseff called to say, "By the way, your brother dated a Russian ambassador's daughter, and apparently, her sister was just arrested in the spy scandal."

It is not known when Kuschenko and Nathan Joseff split up -- but they remain on friendly terms.

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