Actress Bijou Phillips says she knew her half-sister Mackenzie Phillips had consensual sex with their father, Mamas and the Papas leader John Phillips.

In a statement read by Oprah Winfrey on her talk show Friday, Bijou Phillips says she was 13 years old when Mackenzie Phillips told her about the sexual relationship.

Bijou Phillips, now 29, says the news was confusing and scary, and that it was "heartbreaking" to think her family would leave her alone with her father knowing what he did

PHOTOS: Click for photos of actress Bijou Phillips.

Appearing on "The Oprah Winfrey Show" Friday, Mackenzie Phillips said their father "had changed his ways as much as he was able to" and that she felt Bijou Phillips was safe.

But Mackenzie says she did go get her sister when she "felt like she wasn't being watched properly."

Mackenzie Phillips claims in a new book that she had a long-term incestuous relationship with her father, musician John Phillips of the 60’s group the Mamas and the Papas.

PHOTOS: Now & Then photos of Mackenzie Phillips.

In “High on Arrival,” her new tell-all book which hits stores today, the former “One Day at a Time” star says her first recollection of her father having sex with her was on the night before her wedding to Jeff Sessler, a member of the Rolling Stones entourage.

In an interview with People magazine, Phillips, now 49, pleads with readers “don’t hate my father.”

“On the eve of my [1979] wedding, my father showed up, determined to stop it,” Phillips, who was a teenager and strung out on drugs at the time, 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 that it was the first time I was aware of it.”

According to Phillips, the relationship eventually became consensual and continued for ten years.

Phillips also told talk-show queen Oprah Winfrey that it was her father who shot her up for the first time with cocaine when she was just ten years old.

"I remember going into my room, I was crouched on the floor. ... He put the needle in my arm and put the plunger in and he missed," she says. "He missed the vein and my whole arm went numb."

The actress went on to endure severe drug addiction, which cost her role on “One Day at a Time,” divorce and an eventual felony cocaine possession charge in 2008.

Phillip's half-sister Chynna Phillips, who was the front-woman for the band Wilson-Phillips, said she was shocked when Mackenzie told her the news.

In an interview with Us magazine, she says "somebody could have dropped a piano on my head and I probably wouldn't have felt it. But I knew it was true. I mean, who in their right mind would make such a claim if it wasn't true?"

Upon hearing the news, she went into "a deep sadness and depression for about 10 days. A part of me died when I found out."

"After long nights of heroin use, she's claiming that she once woke up and that my father was on top of her having sex with her," Chynna told Us Weekly. "Was he actually raping her? I don't know. Do I believe that they had an incestuous relationship and that it went on for 10 years? Yes."