A friend of mine (male) found out he had a child that was almost four. The reason was because Children Services took her away from her mother and he got a knock at his door one day.
Anyways.... State took all her legal rights away. The Grandparents had temporary custody until he could be located.
So this is the story, and read on because it will give you some reassurance...
Once the dad was found, he immediatley filed for custody. The Grandparents wanted no part in him having her (naturally because he wasn't around.... mind you he didn't know the kid existed until CS knocked on his door).
So anyways....... they did everything to try and affect him getting custody. Even went as far as to say he was a sexual molester. He wasn't but had to go through psychology exams to prove it. It was a long road (almost a year) and he got to know his daughter, and as time went on, his visits were increased to longer, than over night than to days then to five days a week, then to eventually getting custody.
During the custody fight though, the kid knew her sisters father as "Daddy" and the Grandparents insisted it stayed that way. They kept telling her that "Tommy is not your daddy" (Tommy is her bio father).
Tommy and I both always corrected the issue when she was in visits (I babysat for him while he worked). I always referred to him as "Daddy" and when he would pick her up I would say "Daddy is here".
She eventually refrained from calling him "Daddy Tom" to just Daddy.
Needless to say, he was awarded custody and now the Grandparents and mother that refrained him from her are no longer in the picture.
I bet now they wish they weren't so stupid with the situation.
So look up! She will figure it out on her own. Just tell her that you are her true daddy and no one can replace you.