The husband of Kathleen Kennedy was heir apparent to the 10th Duke of where?
William Cavendish, Marquess of Hartington:  He was the husband of Kathleen Kennedy, sister of the future U.S. President John F. Kennedy. Kathleen Cavendish, Marchioness of Hartington:  She was the daughter of Joseph P. Kennedy, Sr. and Rose Kennedy, sister of future U.S. President John F. Kennedy, and wife of the Marquess of Hartington, heir apparent to the 10th Duke of Devonshire.
Devonshire