I am sorry for the passing of your mother in 2016.
If her only income in 2016 (or in previous years) was from SSDI, it is unlikely that a final return was required for 2016.
You can check the attached chart for her 2016 filing status and income. For example, if your mother was single and her gross income was less than $11,900 in 2016, then no tax return is needed.
If her income was greater than that, then you can file a return using the IRS forms and instructions for prior years. Link provided below.
Please let me know if you have additional questions or comments.
https://www.irs.gov/pub/irs-prior/p17--2016.pdf
https://apps.irs.gov/app/picklist/list/priorFormPublication.html;jsessionid=oyRsVXe-LYmF1oYSR5cEvAO9...
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