Everyone has been anxiously awaiting Oprah’s high profile interview with Mackenzie Phillips (set to air today) but the real details behind the “big secret” she’s going to reveal have already been leaked.

During her interview with Oprah, Mackenzie admitted to a 10-year-long sexual relationship with her biological father John Phillips from popular 60’s music group the Mamas and Papas. The shocking details of the relationship (which began as non-consensual but developed into consensual) have been confirmed by Mackenzie’s sister Chynna Phillips.

When speaking of her relationship with her father in her new book “High on Arrival”, Mackenzie reveals the following:

"On the eve of my wedding, my father showed up, determined to stop it," she writes. "I had tons of pills, and Dad had tons of everything, too. Eventually I passed out on Dad's bed."

"My father was not a man with boundaries. He was full of love, and he was sick with drugs. I woke up that night from a blackout to find myself having sex with my own father.

"Had this happened before? I didn't know. All I can say is it was the first time I was aware of it. For a moment I was in my body, in that horrible truth, and then I slid back into a blackout."

John Phillips died in 2001, 4 years after Mackenzie opened about her decade-long relationship with her father to her sister, Chynna.

“She said, 'I don't know why, but I just really felt the need to call you and tell you something that I think you need to know,' " said Chynna.

"[Mackenzie] went on to tell me that she had had an incestuous relationship with our dad for about 10 years."

Mackenzie also talks about her father's role in her drug addiction and how she's coping with life after her father's death.

The shocking (and disturbing) interview airs on Oprah Wednesday (today).