I would suggest using your mother's TurboTax logon that you have used for prior returns that you already have prepared for her. The software will ask you input taxpayer profile information in the subsection "You & Your Family" which is under the "PERSONAL INFO" tab. There will be a specific question in this section, "Do Any of These Apply To ...?". A number of check box responses follows which include "Passed away before filing this return." Check the box before that statement and it opens up a drop down box to input the date of death. You would move forward with the tax prep software as usual. The return would be signed by any appointed representative, surviving spouse, or person in charge of the decedent's property.
The following irs.gov link will provide guidance regarding this matter: https://www.irs.gov/newsroom/how-to-file-a-final-tax-return-for-someone-who-has-passed-away
Sorry for your loss.
