Apparently Lindsay's father, Michael Lohan, had an affair while married to now-ex-wife Dina Lohan. And it resulted in a pregnancy!

Surprise, surprise. Sex leads to pregnancy!

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… [She] is beginning to look a lot like Linds, with a mix of [younger brother] Cody, believe it or not.”

But since Michael hasn't provided financial support for Lindsay's secret sister, the girl's mother says, “It's time for Michael to take responsibility.”

Michael, 48, and Dina, 45, married in 1985 and divorced in 2007. They have four kids - Lindsay, who turns 22 on July 2, Michael, 20, Ali, 14 and Cody, 12.