Help! everytime my nephews come over...

Question

my mom plays the helpless grandma act giving them whatever they want abd going against me whenever i tell them no. like candy and popsicles whenever they wake up and all day long. jr always eats his food at home he would try to pull this b.s. with anyone else but with grandma he takes to bites and starts whining his belly hurts he's full he don't want to eat anymore and she freakin lets him get away with it! and everytime i try to explain the stuff to her she'll be lik why can't he have it? why cant he do this why why why etc and then i'll get to the point where she knows she's wrong and she'll stick her nose in the air and say whatever. i'm not trying to be mean i'm jus trying to make her understand that he is jus like tht around her bcuz he knows he can get whatever he wants from her. i don't know how to explain it to her so that she'll stop. how do i make her see tht wht she is doing is bad and she needs to lay down some ground rules for him and actually follow thru??