I'm very sorry for your loss.
You do NOT need to file. For tax year 2018, you will need to file a return if you are unmarried and at least 65 years of age, and your gross income is $13,600 or more. However, if you live on Social Security benefits, you don't include this in gross income. If this is the only income you receive, then your gross income equals zero, and you don't have to file a federal income tax return.
However, you may still want to file her tax return to establish a date for her last filing. Generally, the IRS can include returns filed within the last three years in an audit. Once that date has past, they will only audit in cases of fraud.
Before you file a decedent return, make sure the Social Security Administration has been notified of the taxpayer's death. You can either go to their website or call 1-800-772-1213.
