A: It is wonderful that you are going to adopt your daughter and that you refer to her as "my 7-year-old daughter" and not your adoptive or not-yet-adopted daughter. She is a daughter in your heart.
Usually, it is easier to explain early to adopted children that you picked them because you wanted to love and raise them as your own biological child. Now it will be a little tricky because your daughter will be surprised and perhaps upset to hear the news. Getting the help of a professional counselor, social worker, clergy person, or family doctor may help make the conversation go more smoothly. In any case, some preparation with what she will have to do or say in court will be a big help to her and to you.
You should be truthful and answer all of her questions, while trying to keep your answers age-appropriate so you don't give too much shocking information all at once. If she expresses curiosity about her biological parents, say that her father has a terrible sickness and cannot take care of her--although he would do so if he could. As she grows older, she may wish to know more information or to meet him. Take it slow, and keep reminding her that you are absolutely devoted to being her father, that you always have been and you always will.