Sept. 25, 2009 -- Mackenzie Phillips is discovering what many who claim they are victims of incest have learned when they come forward -- their families don't always believe them.
The former child star of "One Day at a Time" returned to the Oprah Winfrey Show today to defend her decision to write her bombshell book "High on Arrival," in which she confessed to engaging in a 10-year consensual sexual relationship with her father, rock star John Phillips, the lead singer for the '60s group the Mamas and the Papas.
"It's been heartening and heartbreaking at the same time …with the way my family's reacting," she said about the response to her appearance on Wednesday.
Mackenzie, 49, spoke to Winfrey Friday by satellite. Her half sister Chynna Phillips, whose mother is actress Michelle Phillips, appeared live in Winfrey's studio. Chynna, an actress and singer who is now half of the Christian duo Chynna and Vaughn, said she wanted to appear separately so that her words would not be "convoluted."
Asked about her reaction to her sister's confession on the show, Chynna responded: "I was praying for her the whole time. It's very taboo. Many people were repulsed, horrified, for her to come out and talk about something no one wants to hear about, she had the courage. I'm proud of my sister."
When asked if she believed her older sister, Chynna laughed at first.
"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."
She described the conversation in which Mackenzie first told her about the relationship. Chynna was in an airport, when her sister, with whom she wasn't always close, called her.
"She said, 'There's something you need to know...I had this consensual relationship with dad for over 10 years,'" Chynna recalled. "You could have dropped a piano over my head and I wouldn't have felt it."
Chynna said Mackenzie told her about a month before the book was published that she was planning to go public with the relationship. Chynna said she wished that she had had more time to prepare for her sister's bombshell.
"It's 13 years later and I still haven't digested this information," she said.
Mackenzie responded: "I'm really sorry Chynna to put you in this position, but I don't regret writing the book."
Winfrey also read a letter from Mackenzie's other half sister, singer-actress Bijou Phillips. Bijou was 13 when Mackenzie first told her she had a consensual sexual relationship with their dad. But Bijou said Mackenzie denied it shortly after. She said her older sister's memories are also "clouded by 30 years of drug abuse."
Bijou also wrote that if the man who raised her as a single parent from the time she was three was capable of sexual abuse, "it's heartbreaking to think why would (my family) leave me alone with him."
A teary Mackenzie responded that she never thought her younger sister was in danger of being sexually abused, adding, "I did take her out of there at one point when I thought she wasn't being properly cared for."
Mackenzie also responded to a statement from her stepmother, Michelle Phillips, also a member of the Mamas and the Papas, who told the Hollywood Reporter on Wednesday that she believed the allegations were false.
Michelle said Mackenzie had "a lot of mental illness" and that "she's had a needle stuck up her arm for 35 years."
"When she found out I was writing the book, she did everything to stop me," Mackenzie said. "I don't have history of mental illness, I have a history of addiction. I feel bad Michelle would do this. I think she is trying to protect the Mamas and Papas brand."
Chynna also defended Mackenzie against her mom.
"She doesn't have mental illness by the way," Chynna said about her sister. "I think my mom is angry... It's not something my mom wants plastered all over the papers."
On Wednesday, Mackenzie told Oprah Winfrey that she was first raped by her father in a hotel room when she was 18 while passed out after a drug binge but continued to use drugs and have consensual sex with him for years.
The relationship continued long after she married Jeff Sessler when she was 19, and ended only when she became pregnant and feared her father was the baby's father, Phillips said. John Phillips, she said, paid for an abortion.
Reading an excerpt from her new book, "High on Arrival," on Winfrey's show today, Phillips said of her first sexual experience with her father: "I woke up that night from a blackout to find myself having sex with my father. I don't know how it started."
"For a moment, I was in my body for a horrible truth," she said of waking up and realizing she had had sex with her father.
"I boxed it away. I started very early on in my life compartmentalizing. ... This was the mother of all difficult experiences. ... I [have spent] the last 30 years trying not to look at it," she said.
Mackenzie Phillips told Winfrey she confronted her father about the rape, but he characterized their relationship as romantic, Mackenzie said.
"After the first experience, I went to my father and said we need to talk about how you raped me. My dad said: 'Raped you? Don't you mean made love?' In that moment, I thought I'm really on my own here."
In 1980, soon after she'd been fired from "One Day at a Time" because of her out-of-control drug use, Phillips began to travel with her father's new band, the New Mamas and the Papas. It was during that time, she said, that she began regularly doing drugs and having sex with her father.
"It didn't happen every day, it didn't happen every week, but it certainly happened and happened many times," she said.
RUSSELL GOLDMAN, EILEEN MURPHY and LINDSAY GOLDWERT from ABC News contributed to this report