No, she can not go against the wishes of the parents. Try to be patient and diplomatic with her, usually grand parents always like to spoil their grand children. If she eats sweets once a week, this is normal.

Alhamdulillah my mother in law is causing alot of problems for me. Please i want to know what rights she has when it comes to our children. She always gets angry because i dont like my 3 year old daughter to be eating sweets all the time. Does she have rights to be able to do whatever she wants when it comes to our children even if i dont agree to it?