According to the news reports today, both of Mackenzie's sisters (Chynna and Bijou) have come forward to say that they were aware of the incestuous relationship betwen Mackenzie and their father, and lauding Mackenzie's courage in coming forward publicly with this disclosure.
If the late John Phillips' own daughters believe these allegations against him and support their sister, why should strangers doubt and deride her?
He must have been a very bad man, and all of his children were obviously aware of it.
I would be surprised if Mackenzie were the only one he had sex with... and I not be surprised to learn that the incest actually began at a much younger age than 19. She may have blocked it out.