On September 23rd, former One Day at a Time star Mackenzie Phillips appeared on The Oprah Show to reveal a shocking family secret she’d kept for years. She told Oprah that she’d carried on a sexual relationship with her father, John Phillips, the founder of the Mamas & the Papas, for approximately 10 years.

On Friday, her half-sister, Chynna Phillips, also appeared on Oprah to give her side of the story. Chynna says she learned about Mackenzie’s sexual relationship with John about 12 years ago. Mackenzie told her over the phone, and Chynna says she believes her sister. “Who’s going to go out of their way to say, ‘I had sex with Dad’?” she says. Chynna added that when Mac first contacted her about the relationship she was at LAX getting ready to fly to Syracuse.

Chynna is married to actor William Baldwin whose mother lives in Syracuse. No doubt Chynna was on her way to visit the in-laws when she got the devastating news about her own family.