Complex sentence: Sevier had married a sister of Robert Ward Johnson, a member of a powerful Kentucky political family, who served as a Congressman and US Senator from Arkansas in the antebellum years and was part of the Family.
Split into simpler sentences: Sevier had married a sister of Robert Ward Johnson, a member of a powerful Kentucky political family. Johnson served as a Congressman and US Senator from Arkansas in the antebellum years and was part of the Family.