Complex sentence: Elizabeth Seymour (? 1513-1563) was one of the many children and the second daughter of Sir John Seymour, her sisters being Jane, Margery (who died in 1520) and Dorothy.
Split into simpler sentences: Elizabeth Seymour (? 1513-1563), one of the many children of Sir John Seymour. Elizabeth was the second of the Seymour girls-the others being Jane, Margery (who died in 1520) and Dorothy.