Since the beneficiary of the IRA was the estate and since your mother was over age 70½ at the time of her death, all of the beneficiaries of the will must use your mother’s remaining life expectancy. You will use the Single Life Expectancy Table, look up the age your mother would have been in 2017 (age 84) – the year of her death – to get the factor to use, and subtract one from that factor to get the factor to use for 2018. For each year going forward, you subtract one again to get the new factor for that year.
For example, if the factor for 2017 was 10.2, then the factor for 2018 would be 9.2; for 2019, it would be 8.2, etc. You take that factor and divide it into the prior year-end account balance to get your RMD for the year.
It is important to get the calculation correct. If you do not take out enough each year, the penalty is 50% of what you do not take. Using your own age, as the custodian has suggested, would give you a much smaller distribution and leave you open to the penalty.