I wouldn't say anything about her dad, since as you said, she doesn't know you know.

You could certainly tell her you love her, but just know that she's probably not going to feel comfortable saying it back (well, some T's do), so be prepared for that. It doesn't mean she doesn't care or love you back--many just don't feel it's appropriate to share those feelings with clients.

I recently told my T of almost 4 years that I love her, and she said "That's very nice." But I can tell that she cares about me very much, and that's enough for me. Also recently told my marriage counselor that I love him, but it was more in the "If I love you, is that OK?" sense, so he said that yes, it was OK, but that I had awful taste. It was more that I wanted to know he wouldn't push me away because I felt that. (Things with him are a bit more complicated, with some romantic feelings mixed in with paternal transference.)