In her new memoir, "High on Arrival" and in televised interviews, the former child star alleges shewith her father, the late rock icon John Phillips of the '60s super group, the Mamas and the Papas.
She says it started out as rape but became consensual. Phillips also, thinking the baby might be his.
Phillips also asserts her father introduced her to cocaine. She has a long history of drug problems. He died in 2001 after a lifelong battle with drugs and alcohol.
Her claims are dividing those close to her.
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 is now 29 and says the news was confusing and scary. She says it was "heartbreaking" to think her family would leave her alone with her father.
Appearing as a guest on "The Oprah Winfrey Show" Friday, Mackenzie Phillips said their father "had changed his ways as much as he was able to" and she felt Bijou Phillips was safe.
Mackenzie Phillips says she did go get her sister when she "felt like she wasn't being watched properly."
Also among Mackenzie's supporters are her half-sister, Chynna, friend and former "One Day at a Time" co-star Valerie Bertinelli, and Sr. Drew Pinsky, the Los Angeles psychiatrist who oversaw her rehabilitation in the latest season of his reality show, "Celebrity Rehab with Dr. Drew."
"I know Mackenzie to be practicing a rigorous program of honesty," he told "Access Hollywood." "She would not be sober right now if she were not being completely honest all the time. She may have had distortions about that relationship, but there's no doubt whatever happened severely traumatized her."
Among those questioning Mackenzie's allegations, former stepmother Michelle Phillips, who says in a statement to CBS News, "Whether her relationship with her father is delusional or not, it is an unfortunate circumstance and very hurtful for our entire family"
She also told Us Weekly magazine, "You should take with a grain of salt anything that's said by a person who has had a needle stuck in their arm for 35 years. The whole story is disgusting."
Mackenzie's half-brother, Tamerlane Phillips, is weighing in, as well, saying, "I don't know if these allegations are true are not, (but) they're very surprising to me."
And psychotherapist Robi Ludwig says, no matter what, "In watching Mackenzie Philips, I was worried about her. She seems like a person who is still tremendously tortured by her life experiences, and who can blame her? But I would not say she's out of the woods yet."
On "The Early Show" Friday, former "America's Next Top Model" contestant Lisa D'Amato offered her backing, and explained why, in her opinion, Mackenzie went public with her claims. D'Amato became friends with Mackenzie when they took part in the "Celebrity Rehab" reality show earlier this year:
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