During her appearance on The Oprah Winfrey Show today, Chynna Phillips – the half-sister of Mackenzie Phillips, who recently admitted to a decade-long incestuous relationship with her rock star father John – said she did know about the long-running affair and said she was “proud” of her sister for going public with the revelation, according to published reports. With Mackenzie watching the interview via satellite, Chynna told Winfrey that while people may have been horrified by the news detailed in her sister’s new book High on Arrival, Mackenzie “had the courage.”

When asked by Oprah if she believed her older sister, Chynna laughed and said, “How can the details be perfectly accurate, when you’re on drugs?” she said. “I do know there was a consensual relationship between my father and sister,” she continued. “I’ve known that for about 12 years.”

Chynna Phillips’ mother is Michelle Phillips, a former member of the Mamas and the Papas with John Phillips. Michelle told a Hollywood trade paper this week that she believed the allegations were false and that Mackenzie had “a lot of mental illness.”