Complex sentence: He was the son of John Mordaunt, 1st Viscount Mordaunt, and his wife Elizabeth, the daughter and sole heiress of Thomas Carey, the second son of Robert Carey, 1st Earl of Monmouth.
Split into simpler sentences: He was the son of John Mordaunt, 1st Viscount Mordaunt, and his wife Elizabeth Carey. His mother was Elizabeth, the daughter and sole heiress of Thomas Carey, the second son of Robert Carey, 1st Earl of Monmouth.