N o w - H o l l y w o o d . b l o g s p o t .c o m

In her new memoir, actress Mackenzie Phillips scandalously and disturbingly reveals that she had a 10-year affair with her father, musician John Phillips.

It's an outrageous claim, but likely (and sadly) not an untrue one.

Mackenzie's half-sister, Chynna Phillips, backs her up all the way.

Chynna, of the hit 90s group Wilson Phillips, shares her side of the sad tale, recalling how she once got a call from Mackenzie, now 49, in 1997.

This was 11 years after the affair had ended. Mackenzie Phillips spilled to Chynna, now 41, all about how their father had an affair with her.

"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 Phillips said.

Chynna Phillips believes Mackenzie's story 100 percent.

"[Mackenzie] went on to tell me that she had had an incestuous relationship with our dad (of '60s band The Mamas and the Papas) for about 10 years."

When the affair began, John Phillips was married to his third wife, model Genevieve ­Waïte. Chynna's reaction, not surprisingly, was total shock.

"Somebody could have dropped a piano on my head and I probably wouldn't have felt it," the singer says in an interview with Us Weekly. "But I knew it was true. I mean, who in their right mind would make such a claim if it wasn't true?"

She says the news sent her into "a deep, deep sadness and depression."

In her book, High on Arrival, Mackenzie Phillips also claims she and her father did drugs together and that her father "shot me up for the first time."

Chynna confirms this as well: "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 said.

"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."

Mackenzie Phillips and her father, Jon Phillips.