A truly horrible situation and I'm really sorry that the daughter wasn't able to talk about what was happening to her
I'm glad you spoke to the police - have you tried social services/childrens services as well?
It's possible that they might be able to help, even if it's just to make the father aware that people are watching and paying attention.. it might stop the abuse.
What I do know from my own experience of being abused by my father is that it's incredibly complicated.. I hated mine but wanted him to love me as well and I was brought up in an environment where family were put first.
Loyalty to the family was the most important thing.. to report them would have been the ultimate betrayal and would have led to me being thrown out/ex-communicated.
Thinking about it now, i wish i'd done it but the mind of a child/teen is much much different to the mind of an adult.. to me at the time, it was beyond unthinkable.
What i would really have appreciated more than anything is someone.. anyone to be there that i could speak to and that would simply listen. To know that someone out there cared about me would have meant the world.
Perhaps you could keep in touch with the daughter and make it clear to her that no matter what, if she ever needs you - she can just call you and you'll be there.
xx