The Danish star had a fling with the New York Jets quarterback in 1988 while he was still married to wife Lisa. She went on to have his child, Killian, in 1989 before fleeing the U.S. to live in Italy.
And despite knowing the child was his, the sportsman kept his estranged son a secret from his daughter Brittny for 20 years - until the reality TV socialite's mum bumped into Nielsen at Ivana Trump's wedding last year (09).
Brittny tells New York gossip column PageSix, "Brigitte literally said to my mom at Ivana's wedding: 'I have a son who wants to meet his sister.' Meeting my half-brother for the first time was really very weird and very emotional. The brother I never knew I had - it's a lot to take in... All my life, I've always said I wanted a brother. I definitely want us to be close."
And Brittny is upset that she has missed out on so much time with her brother: "I feel sad that I've had a brother for 20 years that I never knew growing up. He's very tall and blond, and looks identical to Brigitte. He doesn't speak much English because he grew up in Italy. He's really sweet."