Teenager overeating, eats an entire box of cereal?

My sister has a foster kid who is 15 and eats entire boxes of graham crackers, pop tarts and last night ate an entire box of cereal, and a jar of peanut butter with a nail file and hid it under her bed. Any advice you have to figure out what to do would be great. I feel soooo bad for this girl sneaking and eating in the middle of the night. Please help