The former child star details her sick romance with the late The Mamas & The Papas singer John Phillips in her new memoir High On Arrival, revealing she had an incestuous romance with the California Dreamin' singer for a decade.
Her accusations have split her family in two - her sister Chyna Phillips and ex-stepmum Genevieve Phillips, John's wife from 1972 to 1985, have voiced their support for the former child star, while others have been less supportive.
On Wednesday (23Sep09) her former stepmother Michelle Phillips, who was married to John from 1962 to 1970, blasted the recovered drug addict for further spilling the details of her relationship with her dad in a highly-publicised TV interview with Oprah Winfrey, suggesting the allegations were made up.
She told The Hollywood Reporter's Roger Friedman, "John was a bad parent, and a drug addict... (but) if she thinks it's true, why isn't she with a good psychiatrist on a couch? ...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's had a needle stuck up her arm for 35 years."
The actress wishes relations between her and Michelle were better, but admits she can't blame her and other family members for refusing to believe her claims.
During an appearance on America's Today Show on Thursday (24Sep09), Mackenzie said, "I love my family, I've gone out of my way to tell my story in High On Arrival. And our experiences with our dad were very different, thank God.
"I love my sisters, I love my brothers, I love Michelle. I wish that I had all of their support but I don't and that's alright, I understand. This is very difficult, and to be revealed on a public level just like this is very difficult and my heart goes out to them. God willing, as a family we will be stronger when this all dies down."
But Phillips has one person she can always count on - her son Shane, born in 1987, from her romance with rocker Michael Barakan, aka Shane Fontayne.
She adds, "I sat down with him about a month ago, and I said you need to prepare. As a family we need to prepare for some of the hard truths that are going to come out of this book. I sat down and took him through it and he looked at me and put his head on my shoulder and said, 'Poor mama. You better tell dad!' He's an amazing kid and nothing but supportive of me."