Bijoux and mackenzie

Mackenzie Phillips' shocked half-sisters — Bijou and Chynna Phillips — are very upset. Her skeptical stepmother, Michelle Phillips, calls her a liar with mental and drug problems. 

But

Mackenzie is standing strong about her admission to a decade long sick sexual affair with her drug addicted father, John Phillips.

"If I've started a national dialogue that's going to

help people face these issues, then I'm forever grateful," Phillips said Friday during a live broadcast of "The Oprah Winfrey Show."

Winfrey read comments on the show made by Michelle Phillips, John Phillips' former wife and a member of the Mamas and the Papas.

"I have every reason to believe it's untrue,"

Michelle has said, claiming that Mackenzie has a history of mental illness and

drug abuse.

But Mackenzie stood her ground with Oprah.

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"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.

Chynna Phillips appeared live with Winfrey and explained her mom’s reaction. "It's not something that my mom wants plastered all over the papers."

Chynna admitted she wasn’t thrilled about the incest allegation being public knowledge either. "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."

However, she admits knowing for 12 years and says she believes Mac's claim.

But yet another Phillips sib, Bijou Phillips, who has NOT appeared on Oprah, is a disbeliever.  

She's released a statement that says, in part: "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 … ."

What do you think?

Why did she Mackenzie tell Bijou and Michelle that it wasn't true? Was she trying to keep the peace in the family? 

Maybe she just wasn't ready for the fierce family fallout.

More Mackenzie Dish:

John Phillips' pal Davy Jones: "If I'd known about it, I would have killed John"

Chynna and Mackenzie to appear on "Oprah" on Friday afternoon!

Michelle Phillips calls Mackenzie Phillips mentally ill.

Mackenzie Phillips claims that after her abortion, she never let her father touch her

Chynna Phillips believes Mackenzie Phillips is telling the truth.

MacKenzie talks to Oprah about her dad shooting her up with cocaine for the first time.

Photo: Bijou and Mackenzie Phillips. Credit: Getty Images

Posted by:Elizabeth Snead