First, allow me to offer my condolences on your loss.
The following guidance is taken from IRS Pub. 559 Survivors,
Executors, and Administrators:
"Signature. If a personal representative has
been appointed, that person must sign the return. If it is a joint return, the surviving spouse
must also sign it. If no personal representative
has been appointed, the surviving spouse (on a
joint return) signs the return and writes in the
signature area “Filing as surviving spouse.” If no
personal representative has been appointed
and if there is no surviving spouse, the person
in charge of the decedent's property must file
and sign the return as “personal representative.” [Emphasis added.]