Complex sentence: The Habsburg family had ruled the Viceroyalty of New Spain from its establishment until the Spanish throne was inherited by Bourbons, so Maximilian was considered to have more potential legitimacy than other royalty, but Maximilian was unlikely to ever rule in Europe due to his elder brother.
Split into simpler sentences: The Habsburg family had ruled the Viceroyalty of New Spain from its establishment until the Spanish throne was inherited by Bourbons. So, Maximilian was considered to have more potential legitimacy than other royalty, but Maximilian was unlikely to ever rule in Europe due to his elder brother.