If your mother was not past her RMD beginning date, then you do not have an RMD for this distribution.
If your mother was past her RMD beginning date, and she withdrew her RMD (or more) before she died, then you do not have an RMD for this distribution.
If your mother was past her RMD beginning date, and she did not withdraw her RMD before she died, then your RMD is 25% of what her RMD would have been. Her RMD is calculated from IRS tables based on her age and the account balance on 12/31/2023. The broker should be able to tell you what that was.