Chynna Phillips Responds To Mackenzie's Incest Claims

'Who in their right mind would make such a claim if it wasn't true?,' Mackenzie's half-sister says.

Mackenzie Phillips has claimed to have had a 10-year-long incestuous relationship with her father, the late John Phillips of the Mamas and the Papas — and her half-sister, Chynna Phillips, told

that she believes Mackenzie's account.

Mackenzie is set to make an appearance on "The Oprah Winfrey Show" on Wednesday (September 23) to talk about the situation, which she wrote about in her memoir "High on Arrival" (also out on Wednesday). Chynna recalled how she first heard about it.

"She said, 'I don't know why, but I just really felt the need to call you and tell you something that I think you need to know,' " Chynna recalls about the phone call she got from Mackenzie in 1997, 11 years after the relationship had ended. "And she went on to tell me that she had had an incestuous relationship with our dad for about 10 years."

Chynna says that when she heard about it, she went into "a deep, deep sadness and depression for about 10 days. A part of me died when I found out." She added, "Somebody could have dropped a piano on my head and I probably wouldn't have felt it. But I knew it was true. I mean, who in their right mind would make such a claim if it wasn't true?"

Mackenzie also writes in the book that she and her father took drugs together, which Chynna said she also believes.

"They were both doing drugs together. After long nights of heroin use, she's claiming that she once woke up and that my father was on top of her having sex with her," Chynna says. "Was he actually raping her? I don't know. Do I believe that they had an incestuous relationship and that it went on for 10 years? Yes."