Your father's requirement to complete the 2024 RMD moved to you and your siblings upon his death. Therefore, his RMD is zero. If you and your siblings completed his 2024 RMD equally, your RMD for 2024 is 1/3 of what his would have been had he not died. However, the 2024 RMD can be completed by you and your siblings in any combination, so any one sibling could take out the entire amount of the 2024 RMD and the other's would not have to take out any to satisfy that RMD. If you cannot agree on anything a split other that 1/3 each, you'll need to complete 1/3 of the original RMD.
Note that if the 2024 RMD is completed by end of 2025, waiver of the excess-accumulation penalty is automatic and you can just tell TurboTax that you were not required to take this RMD in 2024 (but you would have to add it to your RMD for 2025).
