Complex sentence: Randolph Jefferson (October 1,1755--August 7,1815) was the younger and only brother of Thomas Jefferson, and twins with Anna Scott, Thomas' youngest sister.
Split into simpler sentences: Randolph Jefferson (October 1,1755--August 7,1815) was the younger brother of Thomas Jefferson. He was Thomas'only brother to survive infancy, and was twins with Anna Scott, Thomas' youngest sister.