Mackenzie Phillips's Sisters Speak Out About Incest Claim

Her sisters are upset, her stepmother calls her a liar, but Mackenzie Phillips isn't backing down from revelations that she had a long-term sexual relationship with her father – and hopes she can be the public face of an issue that she says gets little of the attention it deserves.

"If I've started a national dialogue that's going to help people face these issues, then I'm forever grateful," Phillips, 49, said Friday via satellite during a live broadcast of The Oprah Winfrey Show.

The former One Day at a Time star, who revealed her shocking family secret in a new memoir, High on Arrival, appeared for the second time this week on Oprah, this time to face doubts from within her own family about her relationship with her father, the founder of the 60s band The Mamas and the Papas.

'Untrue'

On Friday's show, Winfrey read comments made by John's former wife, Michelle, also a member of the band and Mackenzie's stepmother. "I have every reason to believe it's untrue," Michelle Phillips said, adding that Mackenzie has a history of mental illness and drug abuse.

Mackenzie didn't take the accusation sitting down, although she did admit to a long history of drug use. "When Michelle found out I was writing the book, she vowed to do everything she could to discredit me ... She's either protecting the Mamas and Papas brand or is having a textbook reaction: She's embarrassed, sweep it under the carpet, protect the abuser," Phillips told Winfrey. "It seems so unkind and ungenerous to lash out in this way."

Angry Reaction

Mackenzie's half-sister, Chynna Phillips, appeared live with Winfrey and explained that her mother, Michelle, is angry. "It's not something that my mom wants plastered all over the papers," she said.

Chynna, 41, admitted that she, too, shared some of that sentiment. "Am I exceedingly joyful that my family secret that I told maybe my therapist, my husband and my very best friend in the whole word [is now public]? No."

Coming Clean

Nevertheless, she said, she has known about the situation for 12 years and, while she wasn't certain that her sister could be entirely accurate about the details, believed that the two were sexually involved for a decade. "I was completely stunned," she said when Mackenzie shared the secret. "I walked around in a comatose state for about 10 days."

Another sibling, Bijou Phillips, has released the following statement: "When I was 13, Mackenzie told me that she had a consensual sexual relationship with our father. This news was confusing and scary, as I lived alone with my father since I was 3, I didn’t know what to believe and it didn’t help that shortly there after Mackenzie told me it didn't happen .... "

"I understand Mackenzie's need to come clean with a history she feels will help others, but it's devastating to have the world watch as we try and mend broken fences, especially when the man in question isn't here to defend himself."