Hi op. I think it's good that you've noticed your dd's weight issues and want to tackle it.
I am not going to second-guess anything you have said and just take your comments and motives at face value.
However, I agree with pp who have cautioned against making your views on her weight evident - either through direct comments or restricting food - as any shame or embarrassment will result in lower self-esteem and more desire to comfort eat.
If she has not had much contact with you for four years, you do not want to create a situation where she doesn't want to see you.
Concentrate on showering her with love, doing fun and active things together, involving her with food preparation and cooking, and only having healthy meal and snack options available.
Helping her to feel really good about herself and her relationship with you, providing good food and activities when she's with you is all you can do.
Her mother will already know that her dd is overweight. Either she doesn't care, doesn't see it as an issue or lacks the skills to tackle it. Pointing it out will change nothing and serve no purpose, except give her ammunition to use against you.