A recent study found that in a group of girls followed over 10 years those who felt they had less control of their diet had higher BMI's and gained more weight than their piers.
I have worked with overweight kids for years and I always stress this to parents, the best approach is to model a healthy lifestyle themselves and provide healthy choices in the home. Especially when there are siblings around, its awful to but a child on a "diet" then they feel pressured and different and are more likely to sneak their favourite foods when no one is looking.
The whole family would should be more active and eating healthy home cooked meals together. It also helps to get kids involved with cooking, so they have ownership over what they eat and have fun in the process.