Updated Monday, July 17, 8:30 pm/ET
Posted Saturday, July 15, 5:30 pm/ET

There's a shocking secret behind Christie Brinkley's recent separation from her fourth husband, 47-year-old architect Peter Cook: He was cheating on his 52-year-old supermodel wife with his 19-year-old personal assistant!

That's what sources have told the National Enquirer, who exclusively broke the story on their website on Saturday, July 15. They identified Cook's lover as Diana Bianchi, 19, an attractive, dark-haired aspiring singer from Southampton, New York -- the town where Cook's architecture business is based, just a few miles down the road from where Brinkley and Cook (who were married in Sept. 1996) have their home in Bridgehampton.

Sources say Cook hired Bianchi as his assistant after meeting her in a toy store in 2005, when she was just 18. It wasn't until the young woman's stepfather -- a policeman in Southampton -- angrily confronted Peter in front of Christie at a recent local event, and warned him to "Stop dating my daughter!" that Brinkley learned about her husband's straying. Their separation was announced on July 11; the couple have a daughter, Sailor, 8, and are raising Christie's son Jack, 11, from her previous marriage to real estate developer Richard Taubman.

It's been reported that, after the separation was announced, Christie, along with daughter Sailor and son Jack, flew to California for privacy and to help take care of her mother, Marjorie, 75, who recently suffered a number of strokes.

The Enquirer reports that Cook did not deny the affair after being contacted, adding: "I love my wife, I love my children...and we are still a family." Diana Bianchi's uncle, Joseph Bianchi, told the Enquirer: "It's an emotional time for Diana and she's really dealing with a lot right now."

Rosemarie Arnold, an attorney for Diana Bianchi, tells Star: "Mr. Cook offered her the sun, the moon and the stars. He made her promises he knew he wasn't going to keep. he exploited and manipulated this teenager."

Arnold adds that Diana may sue Cook for "the harm he's done to her, emotionally and physically."