Assuming that your mother reached age 70½ in some year prior to 2017, upon your mother's death, the responsibility to complete her year-of-death RMD became the responsibility of the beneficiaries. IRA custodians generally require that the IRA be transferred to inherited IRAs for the beneficiaries in proportion to each beneficiary's share. Once the inherited IRAs are established, your mother's year of death RMD can be satisfied from the beneficiary's inherited IRAs in any proportion. Each beneficiary taking part in completing the year-of-death RMD should complete Form 5329 requesting a waiver of the excess accumulation penalty as a result of completing the year-of-death RMD late. The IRS invariably grants the waiver when requested under these circumstances. See the instructions on the last page of the Instructions for Form 5329.
If 2017 was the year that your mother reached age 70½, she died before reaching her required beginning date for RMDs and there is no RMD to complete.
Keep in mind that your mother's RMDs from her traditional IRAs are permitted to be aggregated and the total taken from any one of her IRAs, so if she had enough distributed from another IRA to satisfy the aggregate of her traditional IRA RMDs, her RMDs have already been satisfied. If any of the accounts were 401(k), 403(b), 457(b) or federal TSP accounts, RMDs for those accounts are required to be distributed to beneficiaries before any remainder is rolled over to inherited IRAs for beneficiaries.
