Complex sentence: From 1946 he was romantically linked with the recently widowed Kathleen Kennedy Cavendish, Marchioness of Hartington, sister of U.S. President John F. Kennedy, who was also killed alongside FitzWilliam in the crash.
Split into simpler sentences: From 1946 he was romantically linked with the recently widowed Kathleen Kennedy Cavendish, Marchioness of Hartington, sister of U.S. President John F. Kennedy. Kathleen was also killed alongside FitzWilliam in the crash.