@NosferatuORG
Her problem is twofold. She is addicted to the artificial additives and flavorings found in junk food, and she wants to gorge because the feeling to her also feels extremely nice. Both of these don't have to be a problem if accommodated accordingly, i.e eating at one's caloric need or under while eating takeout and Twinkies, or eating a lot of high volume but very low calorie food like competitive eaters do to train while also avoiding weight gain.
But together the food addiction to high calorie food and her need to gorge are going to make her gain, and her obstinate attitude towards anything 'controlling' her food will ensure she won't be that dainty figure anytime soon (or ever.)
Few people like not being able to have whatever they want, but many put up with it for a time for weight loss or health reasons, and/or accept that they can't have whatever anymore in the quantity they want (or at all) and such is life.
But without finding out why she is driven to eat like she does, or even if this is some unconscious drive towards self-destruction (for indeed that does happen in some cases) she will be stuck for the rest of her life in the hell she is creating. Gradually losing the ability to walk for long periods, and to stand, her world is shrinking. Eventually it will be nothing more then the four walls of her bedroom and the occasional trips to the kitchen and bathroom, until she grows to the point that not even that is possible.
I wonder that if Becky and her marry before Amber gets to that point, if Becky will be held liable for neglect if Becky proves unable to take care of her, and either lets her to lay in her own feces or just up and leaves, abandoning her altogether? There is such a thing as caretaker fatigue, and Becky has a risk of becoming depressed.