IRA custodians are not required to inform *beneficiaries* that an RMD is required from an inherited account. They are only required to notify account *owners* that an RMD amount must be calculated for that account. The custodian of the original IRA surely would have notified the deceased (before death, of course) of the deceased RMD requirement. IRA custodians also cannot force out RMDs since it may be possible to satisfy an RMD from a different IRA account of the owner.
The remainder of the deceased's 2016 RMD can be satisfied by a distributions from any of the beneficiaries' separate accounts in any combination. If one beneficiary took a distribution in 2016 from their portion sufficient to complete the deceased's year-of-death RMD, that's covers it. If the sum of all distributions taken by beneficiaries in 2016 was insufficient to complete the deceased's RMD, there is a late RMD for 2016 that must be completed. The beneficiaries taking distributions in 2017 to satisfy the outstanding amount should file Form 5329 with their 2016 tax return to request a waiver of the 50% excess accumulation penalty for having now completed the 2016 RMD. These distributions taken in 2017 will be reported as income on their 2017 tax returns along with their 2017 beneficiary RMDs taken in 2017.
