However, the same day that the Oprah interview was airing, members of Mackenzie's family – including two of her stepmothers, disputed the controversial news.
"I am stunned by Mackenzie's terrible allegations about her father," Genevieve Phillips, who was married to John from 1972 to 1985, said in a statement to Oprah on Wednesday (the alleged affair began in 1979). "I would often complain to John about her overly familiar attitude towards him and he would tell me that that was just her way. John was a good man who had the disease of alcoholism and drug addiction. He was incapable, no matter how drunk or drugged he was, of having such a relationship with his own child.'
And Michelle Phillips, who joined then-husband John in the '60s rock group The Mamas and the Papas and was his wife from 1962-1970, said Mackenzie told her the news in 1997 – and then said she was joking.
"She called me back and said, 'You know I'm joking,'" Michelle told The Hollywood Reporter's Roger Friedman on Wednesday morning. "I said it wasn't funny. Mackenzie said, 'I guess we have different senses of humor.'"
Further, Michelle said that Mackenzie shouldn't have brought her issues to the world.
"John was a bad parent, and a drug addict. But [expletive deleted] his daughter? If she thinks it's true, why isn't she with a good psychiatrist on a couch?" Michelle said. "I think it's unconscionable that Oprah would let her do her show. I have every reason to believe it's untrue. Oprah should be more judicious about who she has on her show.
"Mackenzie has a lot of mental illness," she added. "She's had a needle stuck up her arm for 35 years. She was arrested for heroin and coke just recently. She did 'Celebrity Rehab' and now she writes a book. The whole thing is timed."
"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 said. "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."
For her part, Mackenzie told Oprah on Wednesday that she didn't think anyone would call her words untrue.
"No one will deny it," Mackenzie said. "I'm taking a huge personal risk telling this story and I have to say, I loved my father and I still do and you may find that shocking, but he was a man who lived in a world where the rules didn't apply to him and he tried to pass that on to us kids. I know for a fact that I am the only daughter that this happened to. And I don't know why he chose me to visit his demons upon. I don't know that. I wish that he were here, still alive."
Chynna will have the chance to say her piece to Oprah as well. She is scheduled to appear on "The Oprah Winfrey Show" Friday after canceling, as did Mackenzie, on a scheduled appearance on "The View."
Related Content from AccessHollywood.com:
PLAY IT NOW: Mackenzie Phillips Reveals Incestuous Relationship With Her Father
PLAY IT NOW: T.G.I.F. - Did Mackenzie Phillips Get Her First Heroin Fix From Dad? (September 22, 2009)