If your brother had sufficient income in 2024 or there were taxes withheld form the IRA distribution, you'll need to file a 2024 tax return for your brother. Any tax liability would be an expense of the estate.
If the IRA-BDA was established for the benefit of your brother's estate, it implies that your brother had not designated any beneficiaries for the IRA. However, if your brother did designate beneficiaries, the IRA-BDA has been established and maintained for the benefit of those beneficiaries, not the estate, and the estate has no direct involvement with the IRA-BDA. (Given that the IRA-BDA has not yet been subdivided, I'm guessing that there were no designated beneficiaries, but you need to confirm that with Fidelity.)
If the IRA-BDA is maintained for the benefit of the estate, you can request distributions and use the funds to pay the estate expenses. Such a distribution would be reportable on an estate income tax return Form 1041 (if income to the estate, including these distributions, is sufficient to require filing). If instead the siblings are the beneficiaries of the IRA-BDA, any distributions would have to be paid to these beneficiaries and the estate would need to ask the beneficiaries to cover the estate expenses. Distributions paid to one of these beneficiaries would be reportable by that beneficiary on that beneficiary's individual tax return and taxable to that beneficiary.
If beneficiaries were designated, the IRA must be split into the respective shares by December 31, 2025 for each beneficiary to use their own age for determining RMDs.
