Complex sentence: Like the Thain and Warden of Westmarch, the Master had only nominal authority, though it extended across the Brandywine into the Marish of the Eastfarthing due to family alliances and the commerce between the two areas.
Split into simpler sentences: Like the Thain and Warden of Westmarch, the Master had only nominal authority. The authority of the Master also extended across the Brandywine into the Marish of the Eastfarthing, due to the proximity and the amount of business between the two areas.