I'm so sorry for your loss!
There are several ways you can sign your mother's final return.
If you have been appointed by the courts, you can file as Executor or Administrator.
If you have not been appointed, you can sign "On behalf of the decedent" or "Personal Representative".
Please look at the two links below. The first is especially important if you are claiming a refund on your mother's return. Please look at the paragraph "Filing the Final Return".