Exclusive: Lindsay's Secret Sister Revealed! Her Mom Talks To Star

Posted June 27 @ 3pm/ET

Lindsay Lohan has a secret half sister — and the girl's mom is speaking exclusively to Star!

Kristi Kaufmann says Michael Lohan is the father of her 13-year-old daughter, Ashley.

"I just want to make sure the truth comes out," Kristi, 44, said from her home in Florence, Mont. "Michael is already out there spreading lies."

Kristi says she had a years-long affair with Michael, which started while he was briefly separated from Lindsay's mom Dina. It continued even after the couple got back together.

"He met me in Houston and we met again in New York," she says.

Kristi says she always suspected that her ex-husband was not Ashley's father, and in 2005 she took a paternity test that proved it.

"The only other person I had slept with was Michael," she says.

"My daughter has a right to know who her father is — and it's Michael," says Kristi, who is now married to real-estate agent Daryl Kaufmann.

Ashley is just 18 months younger than Lindsay's sister Ali and a year older than her brother Cody. Sources say Michael, 48, accepts he is Ashley's father and even called her on June 12, her 13th birthday.

In a statement to Star, Michael said: "I have known Kristi since 1983, I hadn't seen her for 12 years. Dina and I were separated and I had seen Kristi, she was in the middle of a divorce. Years later she contacted me convincing me that I was the only person she was with and that she had my child.

"My lawyers will sort this out."