I am having issues with dealing with my grandmother. There's a certain routine I want for my…

I am having issues with dealing with my grandmother. There's a certain routine I want for my daughter and certain ways I fix her stuff. Like her juice I fix it half juice half water and my grandmother puts straight juice in her bottle I don't want my child having all that sugar and when I say the doctor told me this she says oh they don't know what they are talking about. How do I talk to her about this without being rude she's my only babysitter and I don't have the money for daycare