Complex sentence: Lord Charles was the third son of Prime Minister William Cavendish-Bentinck, 3rd Duke of Portland by his wife Lady Dorothy Cavendish, daughter of William Cavendish, 4th Duke of Devonshire and Lady Charlotte Boyle.
Split into simpler sentences: Lord Charles was the third son of Prime Minister William Cavendish-Bentinck, 3rd Duke of Portland and his wife Lady Dorothy Cavendish. Dorothy was a daughter of William Cavendish, 4th Duke of Devonshire and Charlotte Boyle.