Your sister can file the return as the personal representative since she was named as executor in the will.
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.
See https://www.irs.gov/publications/p559#idm140428158283904
