OK! Exclusive: Lindsay's Secret Sister!

Everyone knows that Lindsay Lohan has a little sister Ali, but OK! can now exclusively reveal that there is yet another, secret sister — that even they have never previously known existed!

Linds' father, Michael Lohan, has admitted to OK! that, while married — but at the time separated — to now-ex-wife Dina Lohan, he had a relationship with another woman which resulted in a pregnancy.

In a statement to OK!, Michael says, "Years later the woman contacted me, convincing me that I was the only person she was with and that she had my child."

In fact, OK! has seen letters Michael wrote to the girl's mother where he says that his secret daughter "is beginning to look a lot like Linds, with a mix of younger brother Cody, believe it or not." He also sent his daughter a photograph of himself while he was still in Collins Correctional Facility which he signed "Love Daddy."

Michael, 48, and Dina, 45, married in 1985 and divorced in 2007. They are parents to Lindsay, who celebrates birthday number 22 on July 2, Michael Jr., 20, Ali, 14 and 12-year-old Cody. Dina, Ali and Cody now have their own reality series, Living Lohan on E!, while Lindsay is attempting to revive her career with guest appearances on TV's Ugly Betty and her currently shooting film, Labor Pains.

Michael's secret daughter has had many conversations with her dad, the girl's mother tells OK!. He even called her for her 13th birthday on June 12. But he has never visited her or provided financial support.

"It's time for Michael to take responsibility," the girl's mother tells OK!

For more on this story — including exclusive, never-before-seen photographs of Lindsay's sister — pick up next week's OK!