You are a successor beneficiary. A successor beneficiary must continue the distribution schedule of the original beneficiary, so in this case the RMDs would be based on your mother's age on her birthday in the year following the year in which her mother died, reduced by one for each subsequent year, just as your mother would have been required to take had your mother not died. If your mother did not complete the beneficiary RMD for the year of her death, that responsibility falls to you.
