I was legitimately fat since before kindergarten, and while I was very emotionally sensitive, I wish that someone would have done something for me. I never made the connection in school (meaning elementary and middle school, by high school health I knew of my own accord) that too many calories = extra weight.

All health classes focused on was the food pyramid when I was young. When I would try to diet as a kid, I would end up eating lots of bread (because it was the largest category) and extreme amounts of sugary fruit (8 servings meant 8 fruits, according to my parents).

My parents were not fat and I don't know why they never taught me how to eat right. They just never said anything about my weight until I was over 200lbs in 6th grade (as a 5'1" female), and at that point, they just told me to sign up for sports and assumed I'd lose weight. Well, being a pitcher in softball doesn't burn many calories, and I continued to gain.

I was involved in a weight loss study (won't go into specifics) as an adolescent where I had to see a nutritionist and dietician once a week for six months. It was absolutely invaluable to me to have this information and I began to lose weight steadily, being equipped with the knowledge to do so.

If there is any possible way to get the parents to a nutritionist as a family, DO IT. Phrase it not as a weight issue, but as a healthy lifestyle issue - nutritionists do not focus on weight unless you specifically tell them to. They gave me hard outlines of the nutrients and calories I needed, but ultimately reiterated "it's not a race to the finish line of 'thin', it's a stroll to the finish line of 'more healthy than you once were'".

I wish I knew a better way to help you, but I think your niece may want help down the line, and I hope you're around to nurture that curiosity about nutrition as it seems she is being given a hard hand in life. It's important to be sensitive to her feelings, but it's important to be extremely clear on what is healthy and what is not as well.