I would be stern with her and say no sugar and if she cant listen then she wont be able to take care of him, its really not a hard request to abide. Its why im worried when i have because my SIL's kids are always hyped on sugar and theyre so cranky all the time and i want happy kids. And i know she would feed my kids sugar just to pee me off. Oh not to mention her 3 yro has 3 missing teeth because his teeth were weak and he smacked his mouth running into a wall, he had no enamel on his teeth