How old is your daughter? Has the doctor said she is overweight? Could she just have a different body type than the rest of the family. Looking back on your previous questions, I'm thinking she's a tween.
Kids do fill out before they grow taller. Could this be happening with her. My granddaughter is now 10. She has always been a very thin child but she is beginning to fill out by getting wider and thicker. Because she was so thin earlier I could think she was getting to fat but because she's still growing taller, none of us are concerned.
I'm glad you're wanting to eat healthier foods. This is good for the whole family. and getting into having consistent exercise is also great for the whole family. I would not focus on this one child.
I also agree with drinking a glass of water before eating. Have the whole family do that. This will help to fill up her stomach. It won't hurt anyone else. When we are in the habit of over eating we stretch out our stomach which causes us to continue to eat more. You might talk with her, if the doctor suggests she needs to eat less, about the way her digestion system works.
My daughter was obese at 10 or so. We took a nutrition class at the hospital. The focus was not on losing weight. It was on learning which foods are best for us and how much of what we need to eat. ie the food pyramid. This could help the whole family, even if you're the only one who takes the class.
I'm impressed with the numerous great ideas. Both exercise and a healthy diet are wonderful for the whole family. At home is where we begin the habits that last a lifetime.