Chynna Phillips has confirmed that her half-sister carried on a long, incestuous affair with their father, John Phillips.

Now, as disgusting as it is to contemplate, a long-time friend of Mackenzie Phillips is also backing up her pal's story.

Owen Elliot, the daughter of John’s bandmate Cass Elliot, told Extra this week:

“Mackenzie is my best friend. She speaks her truth, and it’s not a pretty truth. She has lived with this for 30-plus years, and talking about this now is an important part of her recovery. I am very proud of her. Her bravery in telling her story is bound to help others.”

Whether or not it's true, it's also bound to make her a lot of money.

Are you grossed out by Mackenzie Phillips, or fascinated by her tale?

Her sisters are upset, her stepmom calls her a liar, but Mackenzie Phillips isn't backing down from her claim of a long-term sexual relationship with her dad.

She hopes to become the public face of incest, not a post anyone would want to hold, but one that represents an issue she says gets too little attention.

"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 on Oprah.

Mackenzie Phillips, who revealed her shocking secret in a new memoir, High on Arrival, appeared for the second time this week on The Oprah Winfrey Show.

She returned to Oprah to face the doubts from her family about the nature of her relationship with John Phillips, founder of the band The Mamas and the Papas.

Bijou, Chynna and Mackenzie Phillips.

On Friday's show, Winfrey read comments made by John's ex-wife, Michelle, who was also a member of the band and later 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 Phillips, who also says she became pregnant, possibly with John's love child (which she aborted), didn't take the accusation sitting down, although she has by no means attempted to deny her long and troubled history of drug use.

Seemingly out of nowhere week, we saw Mackenzie Phillips on Oprah, revealing a long held family secret - that she had a 10-year sexual affair with her father and possibly even became pregnant by him before having an abortion.

Well, we haven't seen the last of the 49-year-old on the daytime talk show. Mackenzie is returning to The Oprah Winfrey Show tomorrow (Friday) afternoon.

Joining her? Half-sister Chynna Phillips, who spoke out this week in support of Mackenzie, who she says confessed the affair to her 11 years after it ended.

Mackenzie will appear via satellite on Friday, while Chynna will be live in the studio. It is not clear if the two will interact or if they'll appear only separately.

Mackenzie Phillips, left, and Chynna Phillips.

On Wednesday's show, Mackenzie confessed her long-running, drug-addled sexual affair with her dad, John Phillips of '60s band The Mamas and the Papas.

She said she became pregnant and didn't know who fathered the child. She had an abortion, which her father paid for, "and I never let him touch me again."

Chynna later revealed her side of her sister's sad and disturbing story, which Mackenzie is making public for the first time in her new memoir, High on Arrival.

In an interview, she said that news of the affair sent her into "a deep, deep sadness and depression for about 10 days. A part of me died when I found out."

It will be interesting to see what transpires on Friday's show.