How do i tell my mil nicely not to do somthing?

basically i have a 21 mth old son from a previous relationship although my partner and his family have always treated him as there own (which is great seeing as the father and family dont bother ) i love my MIL shes great (better than the last one ) she cant do enough for me and my son but its starting to get abit much and she wont listen when i say no ! my sons a great eater but when shes around she gives him biscuit after biscuit he eats a full packet i tell her no hes getting his dinner soon and its not good for him and she says it wont do him any harm its starting to get on my nerves to the point i avoid her calls and avoid calling because now my son thinks its ok to rid the sweet cupboard and its so hard to get him into a good eating routine after all the sweet shes given him the worst is i feel like screaming at her your not his nana stop it ! its got more annoying because my son calls my partner dada which we did not encourage he picked it up him self but she keeps saying go to garath son ive tried to explain that she has now confused the child and it upsets him it feels like she does it on purpose help !!!!!!