The character sheet for Princess Luna in my AI Dungeon space opera fanfic, 3343: Spaceship Runners, describes her as serving as regent to her older sister, Princess Celestia, also known as Empress Starluna. She holds a position on the Regency Council within the interstellar feudal empire of Bhaattirnia, where her older sister rules as Empress.

Lore from the story card:
“Princess Luna, the younger sister of Princess Celestia, is an alicorn mare who serves as a key leader of The Regency Council in the Integrated Regency of Bhaattirnia, utilizing her exceptional magical abilities and diplomatic skills to maintain harmony among the council’s noble representatives, while navigating the intricate web of alliances and rivalries within the Virgo Supercluster, driven by a desire to protect the regency’s interests and promote cooperation among its member worlds, and often seeking to balance her own ambitions with the wise counsel of her sister, Princess Celestia, in the governance of the regency and the broader geopolitics of the supercluster.”