I agree with the ideas about higher protein foods which are more filling. If she eats too many carbohydrates (pastas, breads, rice, etc.) cut those way down; they make blood sugar rise and then it crashes and you feel hungrier later than if you eat proteins and fiber.
Also, be sure you only put limited portions on the plate (and on your own and your husband's, so she cannot say "Why do you get more?") and no seconds except on vegetables, for instance, never on carbs. To prevent a fight over "I want seconds," ensure that the meal is very focused: You all sit down, you talk and share while you eat, but you do not watch TV while eating (a huge no-no; it distracts us and we eat more); and when the meal is over it's over swiftly -- don't keep staying at the table, ensure she clears her plate and the other plates (make it her new chore) and be certain she has something she MUST start doing immediately after dinner! If her homework is done before dinner, then always have something she needs to do as soon as dinner is done, even if that something is a walk with you, or a chore, or reading a book. Don't let her linger when the meal is done.
Waiting 20 minutes to feel full is right, but kids don't want to listen to their bodies' "full signals" so having something that must be done when the meal is swiftly ended can help distract her.
Also ensure that she is not eating for comfort or for fun because it's what she does when at home -- she should have other activities at home so she doesn't associate being in the house with getting another snack.
Limit what foods you keep on hand. If she wants to snack all the time, do not have snacks available other than fruit and water etc. She cannot eat what is not in the house! Your husband and her sister might object but you can sell your husband on the idea that your doctor says it's important to watch her intake, can't you?
If she is now allowed to go get her own snacks when she likes, end that, even if you end up having to lock the pantry or cabinet door. (Yes, that sounds harsh but some families do this if a child tends to go get food on his or her own constantly.)
Does she buy school lunches if she's in school? If so, I'd cut that out. School lunches might say that kids have to take one veg and one fruit etc. but the reality is (and I've seen it while visiting my kid's cafeteria) that kids TAKE those things because they must but then throw them out because they fill up on the pizza, burgers, tacos and starchy junk. Pack her lunches and involve her -- appeal to her sense of independence and take her shopping for lunch box foods and guide her toward the veg and fruit you know she likes. She is not too young to learn to cut up a cucumber with supervision, or make a roll-up from half a whole wheat tortilla and light cream cheese....