12yr old who wants to lose weight

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My daughter is very obese and says she wants to lose weight. I am overweight myself and her father is now extremely overweight, when she is at my house i carefully monitor what she eats and shes active, while at her dads (every other weekend) its a total junk food fest. She tells me all the time she wants to lose weight and i feel like i do what i can to help her, we walk each night, play wii dance games, she lives in the pool, she plays every sport at school but here is the problem...she likes to sneak food! i dont buy alot of junk food but there is some in the house, there are 3 other teens too. The snack food i buy for them would be yogart, fruit, granola bars (100 cal kind), and rarely fruit snacks. Their favorite thing is tuna so i buy alot of it the problem is she will eat the whole can herself. How do you help a kid who is upset with the way she looks, tells you she wants to lose weight but will not change her eating habbits? I try to lead by example, i weigh out all my food and do the same with all the kids so she doesnt feel isolated but i just feel like i am at a loss. any advice will be greatly appriciated!