What a wonderful post, Bjork! How these little girls must love their Aunt Sunshine.
Are you still seeing a therapist for your eating disorders? I would start there, because this person would be well-equipped to help you make some very individual choices about your own behavior. Likewise, s/he would be able to say, "Bjork, you have a healthy outlook and are a good example for these children now!"
Yes, we want to pay attention to ourselves and live healthfully so we will be a good example to the innocents. When they are older, they will be your companions in good eating, and you will enjoy your meals together. I know this from experience. There were/are a lot of eating disorders in our family. One sister died with bulimia, one was depressed and overweight, and I was overweight for many years with severe co-morbidities. The aunts, OMG, every time these large women looked at any of us, there was a comment on our weight problems! Yes, we do want to stop all that nonsense here and now. The beauty you see in your nieces is a reflection of yourself. The love, the same.
Now I am slimming, nearly normal weight after pretty radical surgery, and it gladdens my heart that my son (who was a roly-poly pre-teen) is as attendant to his nutrition as I am to mine. We used to have our little feeding frenzies. Now, that's not even a thought.
Never give up. I think the love you have for these little girls will be just the thing to keep you moving forward with your own struggles. What a lovely thing to do for them: find the examples you want to give them, and reinforce your own healthy behavior so that you'll have a long life to spend with them.
Sounds like a win:win proposition to me.