Phillips, 49, claims in her new book, "High on Arrival," that she and her late father, the Mamas and the Papas musician John Phillips, had a consensual sexual relationship off and on from the time she was 19 to 29.
While Mackenzie's half-sister, Chynna, has offered her support, her stepmother, Michelle, has categorically denied the allegations.
McCready told "Access Hollywood" Mackenzie let her read an early copy of her new book.
Asked if she believed everything Mackenzie wrote in it, McCready said she "absolutely" believed her.
"Nobody has the right to say what they do or don't believe unless they know her," McCready said, adding she has spoken to Mackenzie since her book came out last week. "I just encouraged her because I knew what was going to come out. I knew that her family was going to be upset and the underlying thing going on during 'Celebrity Rehab' and that taping were some of those things. The family was finding out that she was going to discuss these things and, of course, no one wanted her to discuss them."