I'm going to agree with the others that believe the therapist was working in the girls best interest as well as yours. If she is a family therapist then she can't take your side just because you had good intentions. Perhaps the therapist wants you to help the children find their voice. I know the girl asked for you advice but perhaps her first draft would have been good enough for her father. Personally, I'm old school and wouldn't feel comfortable reading such a personal letter. it's probably intimidating to her just knowing the guards are reading it.

Best of luck to you all and I just want to say that you sound like a great Aunt. Not only are you providing for the child but you understand the effect that this situation is having on everyone involved. Good for you.
That's has to be hard on you. Take care of yourself.