First, S., you are right to be concerned, you are a good parent to be taking action, and get that thyroid checked! SM's ideas are terrific.
Also, get your stepdaughter out of the house immediately. Is she in any summer activities? That doesn't have to mean camps, though it can. Is she away from her normal friends if she's with you "for the summer" only? If so, she needs activities with other kids to keep her from being bored and eating, plus she needs physical action -- exercise not only burns calories, it actually can suppress appetite for a while after exercising. How about lessons (what's she interested in? Tennis, swimming, soccer; maybe something more unusual like archery or fencing? If it interests her, go with it)? Or even arts or crafts lessons -- try your local community arts center or county recreation centers. She needs to be occupied and interested. Can you take her to the playground regularly?
While at home, she also might respond well to having more responsibilities around your house. Don't make her feel this is punishment--make it an honor for her as the big kid. Find something she really would like to take responsibility for, like helping with your toddler or weeding the garden and then planting gorgeous flowers she made a special trip with you to choose for herself (that's physical activity!) etc. Maybe pay her a little for "extra" chores that go above and beyond keeping the house going.
When she eats less or helps more, give her lots and lots of praise and attention. Even though you and your husband clearly pay attention to this young lady, in her mind it may still be hard being the big kid when there's a cute and attention-getting 2-year-old around. Is it possible she's stressed being away from her mom? Even if the relationship with her mom is in some way tough for the girl, she may still feel out of place and on her own, even if you and your husband are close to her. Can you talk with her about this?
The actions of shoveling food into her mouth from dishes in the kitchen literally while the family's still eating in the other room are worrying. You may end up having to remember to put food away quickly. I'd put all leftovers that I could into the freezer --it's harder to stuff yourself out of the freezer than the fridge! Since you're clearly aware of her tendency to chow on lots of what's left, cook a lot less and make it her job to put things away in the freezer or fridge (with you in the kitchen, not hovering or scolding but watchfully busy doing something else a the same time).
And do consider whether counseling is in order if all this doesn't work. Will she stay with you in the fall or return to fast food at her mom's? That's a big issue, I think. If you get her on a good track this summer, consider how to help her continue it. Good luck.