The IRS is not picky. If you have not been named the executor by a court, the IRS will consider you a personal representative if you are the person (or one of the people) who is in constructive receipt of the deceased person's property. You don't normally need a court order. (In cases where there are more than one heir or there is a dispute between heirs, a court might be needed to resolve the dispute.)
See here,
https://www.irs.gov/individuals/file-the-final-income-tax-returns-of-a-deceased-person
https://www.irs.gov/forms-pubs/about-publication-559
A personal representative of an estate is an executor, administrator, or anyone who is in charge of the decedent's property. Generally, an executor (or executrix) is named in a decedent's will to administer the estate and distribute properties as the decedent has directed. An administrator (or administratrix) is usually appointed by the court if no will exists, if no executor was named in the will, or if the named executor can't or won't serve.
In general, an executor and an administrator perform the same duties and have the same responsibilities.
For estate tax purposes, if there is no executor or administrator appointed, qualified, and acting within the United States, the term “executor” includes anyone in actual or constructive possession of any property of the decedent. It includes, among others, the decedent's agents and representatives; safe-deposit companies, warehouse companies, and other custodians of property in this country; brokers holding securities of the decedent as collateral; and the debtors of the decedent who are in this country.
