I am sorry for your loss.
Since you are the personal representative (executor), you might want to read through the IRS publication at the link below.
https://www.irs.gov/publications/p559#en_US_2021_publink100099488
Note that, first, you will file a final return (1040) for your mother that would cover the period from January 1, 2021 until the date she passed in May.
