Advice on how to get my daughter to lose weight

Shes 12 and about 25-30 lbs overweight. She wants to lose weight so we have been trying to incorporate no ore workouts( through youtube sports are not open due to pandemic).

She does to her grandmothers house every other weekend Tuesday nights and breaks (court ordered).

I am vegan and eat 100% plant based and only give her plant based foods while she is with me however when she is with her grandmother she overfeeds her and gives her all tupes of sugar meat(which i dont approve of) diary etc. I have spoke to her grandmother and her dad (who is not too involved) about giving her meat and dairy and her grandmother has even agreed (she used to be vegetarian so she knows how to cook without meat). I have even brought over food for her to eat but they refuse to give it to her.

I am not trying to blame anyone but it would be helpful if i had support. My daughter will eat anything that i dont allow her to eat just because thats what she likes but im afraid she will become obese if we dont get her weight under control. She gains weight very quickly.

Any good advice?? Thanks!