Complex sentence: Burke's Peerage Sir Thomas Seymour, 1st Baron Seymour of Sudeley, and Elizabeth Seymour, Marchioness of Winchester; and were thus great-Grandparents to King Edward VI.
Split into simpler sentences: Burke's Peerage Sir Thomas Seymour, 1st Baron Seymour of Sudeley, and Elizabeth Seymour, Marchioness of Winchester. And thus, great-Grandparents to King Edward VI.