Is there a sin or an expiation for an oath (kaffarat yameen) incumbent upon someone who swore to her deceased father that she would travel to him to bring him, along with her husband, to her home, knowing that she was unable to fulfill this due to his death three years after the oath?

Allah has commanded kindness and good treatment towards parents, and this kindness is contingent upon one's ability. If you are unable to fulfill your father's wish, there is no sin upon you. As for the oath, expiation is required of you, whether the breaking of the oath was due to an excuse or not.