You cannot file the Final return (a form of T1-General for deceased) or the Trust return (T3) with the POA. The POA becomes invalid on the day of death. If there is no will you have to file for the affidavit otherwise the return will be subject to post-assessment or audit by CRA. CRA allows one person to file for the affidavit (form RC594E) for the purpose of filing the tax returns for the deceased.
The reason you need this affidavit is that someone has to sign the return on behalf of the deceased, and POAs are given by the deceased while he/she was alive and invalid on DOD, so CRA needs another official form to allow a family member to sign the tax returns on behalf of the deceased even if he/she is the only relative alive.
I hope this was helpful
