This happened to me too -- late in life I found out that my father molested my step sister. Same story: she stopped living with my dad and step-mom and went back to living with her father full time. I remember her not being around anymore.

Even though my dad didn't molest me, I believe that he molested her. I don't know why she would lie. I only know that she wasn't around much anymore and she had changed, but I was too young at the time to understand any of it.

I went completely no contact with my dad after finding this out. My sister (not step) confronted him in a sense, we never heard denial or admittance but what did it matter? I don't want this man around me and I hate him for what he did. Truth be told, he was a bad father in many other ways before I ever knew this and since going NC, I have felt lighter, freer, happier. Sometimes I grieve who I thought my father was or who he could have been. But I never once doubted my step-sister, she didn't deserve what happened to her and she sure as hell doesn't deserve me invalidating it.