I agree. I think that Petunia will have great difficulties to change and I also believe that the way she treats Dudley is primarily selfish. In the books I seem to recall that she does make an effort to put him on a diet, doesn't she (making Harry follow the same regime)?

It's an interesting question, though, whether parents who are incapable of restricting their children's food intake are to be considered abusive. On the one hand extreme overweight is a considerable health risk - you could argue that the children's health and life are in danger. On the other hand, at least in my country, I don't think that it has ever happened that a child has been removed from their guardians on those grounds. Maybe it occurs in other countries?