William Seymour, 2nd Duke of Somerset was the great-grandson of Edward Seymour, 1st Duke of Somerset (c.1500-1552), the uncle of King Edward VI and Lord Protector of England. Seymour made a secret marriage at Greenwich on 22 June 1610 to Arbella Stuart (d.1615), daughter of Charles Stuart, 1st Earl of Lennox and Elizabeth Cavendish. Arbella was thirteen years his senior, and King James I disapproved of the marriage as the union of two potential Tudor pretenders to the throne, who were respectively fourth and sixth in line, could only be seen as a threat to the ruling dynasty. As a result, William was condemned to life
Frances Seymour, Duchess of Somerset thus incurred the Queen's wrath. Essex was executed on 25 February 1601 for allegedly conspiring to murder the Queen. He left behind his wife and small children; Frances was less than two years old at the time. Frances's brother was Robert Devereux, 3rd Earl of Essex, and she had an elder sister, Dorothy. She also had three younger half-siblings from her mother's third marriage in 1603 to Richard Burke, 4th Earl of Clanricarde. These were Honora, Mary and Ulick Burke, 1st Marquess of Clanricarde. On 3 March 1616, Frances married at Drayton Bassett William Seymour, 2nd Duke of Somerset (1588-1660),