""Her build is just so different to her older sister and they both have the same diet!! I am thinking portion control may be a factor though.""
Children are individuals, I don't lose weight, healthily through portion control. Some people just can't eat what others can and not be overweight.
She's too young to be fed differently from her sibling, but different levels or types of exercise might be the difference. Is she interested in martial arts etc?
I was an obese child and anyone who was will tell you that it takes years to undo the damage of a bad relationship with food and exercise.
The research is showing us that the way high fat/sugar foods "comfort" our brains, isn't something that we want to start in children, because the need for this comfort, stays with us and who wants to give their children daily battles (even as adults)?
For me and two of my DDs, weights are the only exercise that has made a dramatic difference. Which is more suitable, later on.
It's interesting that when you start to eat as we should be eating, there's a lot of shops that you'd only go in to buy toilet rolls, or frozen spinach, Iceland, Farmfoods etc. As for Tesco/Asda, even Aldi/Lidl, you only shop in a third of the store.
I live in a disadvantaged area, my Tesco is stocked differently than one in a more wealthy/better educated area, because they stock what sells.
OP, you've been given an opportunity that I wish my Parents were forced to take. Find with your DD what works for her body.
Most of the 'food' sold and advertised doesn't work for anyone's body. Coupled with the lack of exercise, we are facing the issues that we are, as a society.