Missed RMD Where IRA Owner and Spouse Beneficiary Both Passed Away in Same Year

My father-in-law was the owner of a traditional IRA that he had been taking RMDs from. He passed away in June 2021, at the age of 96, prior to withdrawing the RMD for 2021. His spouse (my mother-in-law), the beneficiary of the IRA, passed away in Sept 2021, at the age of 82, unaware that the IRA existed. My wife, the executor of my mother-in-law's estate, first learned of the IRA's existence in Mar 2022. Upon learning of the IRA, my wife contacted the IRA custodian, and processed a distribution to claim the IRA on behalf of my mother-in-law's estate. My mother-in-law's estate is now listed as the IRA owner. Prior to distributing this IRA to the estate beneficiaries in the form of inherited IRAs, she thinks she needs to satisfy the missed 2021 RMD. Is an RMD required in this situation, how is the amount calculated, where is the distribution reported, and who pays the tax on the distribution?