if you think its food from the pantry that she is helping herself to, maybe don't buy it so the temptation isn't there. I know you said it might be a genes thing...when you say you and your dh and big, I am not sure whether you mean big build (as in tall) or as in big meaning extra fat. Keep a food diary for her so you can see whats going on and you can give that to a nutritionist. Sometimes even eating healthy foods can cause problems if their body can't cope with them in some way. eg bread. Hope you can find the support you need for your kids.
All the best with it!