Grandparent take care LO

Quite angry with my Mil when she insist her style of taking care the baby and ignore what we told her.. but i cant do anything coz i need her help to take care my LO when i go to work... example she always like to prepare the formula 30-40mins ahead of the feeding time although i hv repeatedly told her not to do so. I said the formula might spoil, but she jus do as what she like.. sigh..do know how...stuborn old ppl!