Complex sentence: The Scott family held half of Branxholm, a Teviotdale estate, when Sir Walter extended his ownership to the other half through an exchange with Thomas Inglis, which put the Scotts of Buccleuch in position to rival their feudal superiors--the Douglas.
Split into simpler sentences: The Scott family held half of Branxholm, a Teviotdale estate, when Sir Walter Scott extended his ownership to the other half through an exchange with Thomas Inglis. This significant deal put the Scotts of Buccleuch in position to rival their feudal superiors--the Douglas.