Complex sentence: When Cavendish's older brother, William, Marquess of Hartington, was killed in combat in 1944, Cavendish became heir to the dukedom and Marquess of Hartington; in 1950, upon the death of his father, he became the 11th Duke of Devonshire.
Split into simpler sentences: When Cavendish's older brother, William, Marquess of Hartington, was killed in action in 1944, Cavendish became heir to the dukedom and Marquess of Hartington. In 1950, on the death of his father, Lord Andrew became the 11th Duke of Devonshire.