She refuses to eat veggies! now anemic?

Shes my neice i live with her and her mother and her grandparents. She eats mcdonalds ice cream and thats it. She will eat carrots with ranch but thats it. I cant mix veggies with stuff she will pick them out with out even trying them. If shes sees green or red or any color she doesnt like she wont eat it she is 8 and getting fat. The dr told her mother she was anemic and needed to start eating veggies but her mother doesnt even try. I tried yesterday but she just cried and her grandpa just held her and got her to drink oj which was less then 1percent fruit juice and was talking so much crap about 8year olds mother saying she only buys sugary drinks. But he does to. I buy healthy food all around and only 100percent fruit juice but she refuses to eat it. I need help. If i try to force her to stay in the kitchen she will cry and someone will rescue her. She is not my daughter but i do love her and im getting worried. If she becomes diabetic noone will make sure she eats right. How can i secretly hide these veggies and make her eat them. Cant blackmail her or ground her noone will follow through