Inherited IRA

My husband and his sister are beneficiaries of their mother's IRA who passed away early January 2022. My husband took his half in a lump sum in 2022 and his sister took her half in a lump sum in January 2023. My husband got a 1099-R for his portion and TurboTax is asking us how much applies to the RMD that needs to be taken out. How do we figure this out?

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