The taxable amount shown in box 2a of the Form 1099-R is income to your father's estate, reportable on an estate income tax return Form 1041, including Schedules K-1 showing this Distributable Net Income as having been passed through to the estate beneficiaries for inclusion and taxation on the beneficiaries' tax returns.
Assuming that the estate uses a calendar income tax year, this is reportable on a 2025 Form 1041 which has a due date of April 15, 2026 (unless a filing extension is requested providing an automatic 5½ month extension) and the income passed through to the estate beneficiaries will be reportable on the beneficiaries' 2025 tax returns. The estate will take a deduction for the income distributed to the beneficiaries, so the estate will not owe any taxes.
