From PEOPLE Magazine Click to enlarge
When Mackenzie Phillips wrote her new memoir High on Arrival, she knew the family secret revealed within would detonate an explosion: For years, she wrote, she was sexually abused by her father, John Phillips, founder of the Mamas and the Papas. Sure enough, days after her book (excerpted in PEOPLE, Oct. 5) was published on Sept. 23, the actress, 49, was facing the reactions of her jolted family. In a split-screen appearance with Mackenzie on Oprah, half sister Chynna, 41, praised her for having "the courage to come out and talk about" her past. Half sister Bijou, 29, released a statement saying she found it "devastating to have the world watch...especially when the man in question isn't here to defend himself." Phillips's stepmom Michelle, also of the Mamas and the Papas, was furious. "I have every reason to believe it's untrue," she told Showbiz411.com. "Mackenzie has a lot of mental illness. She's had a needle stuck up her arm for 35 years."

Hurt but undaunted, Phillips shrugs off Michelle's talk of mental illness as "very excessive" and instead hones in on the attempt to discredit her incest story. "Her claims that I'm a liar, I find them shocking," Phillips tells PEOPLE. "Conversely, I understand. It's difficult and embarrassing." It's also, the former One Day at a Time star is learning, a secret shared by countless others. Phillips says she's received "hundreds of letters of people saying, 'Now I know I'm not alone.'"

Though she's incurred the wrath of two of Papa John's four wives (Wife No. 3, Genevieve, told Winfrey, "He was incapable...of having such a relationship with his own child"), her own mom, Susan Adams, John's first wife, says, "I'm so proud of her." Chynna dismisses her mother Michelle's charge of mental illness, saying it's "unfair" to assume a drug addict is mentally ill.

Phillips is learning that putting her name on so incendiary a topic incurs a weighty responsibility. Her mention of a "consensual" relationship prompted many letters asking that she not use that word. "Maybe I did a disservice in not doing research on incest," Phillips says. "I'm learning a lot about the fact that consensual incest doesn't exist. It's participation, it's cooperation."

And, of course, it's taboo. "Incest is such an isolating crime," says Jennifer Wilson of the National Sexual Assault Hotline run by the Rape, Abuse & Incest National Network (RAINN). "Mackenzie's talking about this is going to be empowering for a lot of survivors." It may most help those whose sexual relations continued into adulthood. "An older child is guilted into believing it's something they should do," says Dr. Annette Hernandez, a psychiatry professor at the Mount Sinai School of Medicine. "There's a dynamic that alters the person's perception of what is right and wrong."

Phillips's own perception is changing. "'Knowing better' doesn't mean you feel you have the power to tell somebody or to walk away," she now says. As for speaking out, she quotes one of the thankful e-mails she received and says, "I have no regrets whatsoever."