In general, the personal representative of a deceased person has the duty to file a final tax return. That would be the executor, or some other terminology in your state. If you are not the personal representative, you may need to go to probate court to get appointed.
The person's debts and taxes must be paid before the rest of their estate is distributed to family or other heirs. If your mother owes tax and it is not paid before the rest of her estate is distributed, the IRS can collect the taxes from the heirs, and while the IRS normally has 3 years to audit a taxpayer, that clock starts when the return is filed, so if a return is never filed, the clock never starts.
It is possible that no tax return is required, if the person's only income is non-taxable social security, or their other taxable income is less than their standard deduction. That will depend on her particular situation.
