Chynna Phillips, the half sister of actress Mackenzie Phillips, says she is telling the truth about an incestual relationship with their father, John Phillips.

Chynna, 41 says her sister called her in 1997; 11 years after the affair ended to tell her she had an incestuous relationship with their dad for about 10 years. Chynna also says they both did drugs together, according to US Magazine on Wednesday.

Somebody could have dropped a piano on my head and I probably wouldn't have felt it. But I knew it was true, Chynna said, US Magazine notes. I mean, who in their right mind would make such a claim if it wasn't true? she adds.

Mackenzie unveiled this 31-year-old secret in her new memoir High on Arrival and in an episode of the Oprah Winfrey Show airing today. Her father founded The Mamas and the Papas band of the 1960's and also introduced her to cocaine, she claims.