Question: What American Socialite married the elder son of Edward Cavendish?

Context paragraphs:
Lord Edward Cavendish: Lieutenant-Colonel Lord Edward Cavendish (28 January 1838 – 18 May 1891) was a 19th-century British politician, soldier, and nobleman.

West Derbyshire by-election, 1938: The West Derbyshire by-election of 1938 was held on 2 June 1938.  The by-election was held due to the succession to the peerage of the incumbent Conservative MP, Edward Cavendish.  It was won by the Conservative candidate Henry Hunloke.

Kathleen Cavendish, Marchioness of Hartington: Kathleen Agnes Cavendish, Marchioness of Hartington ("née" Kennedy; February 20, 1920 – May 13, 1948), also known as "Kick" Kennedy, was an American socialite.  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.

Baron Blayney: Lord Blayney, Baron of Monaghan, in the County of Monaghan, was a title in the Peerage of Ireland.  It was created in 1621 for the soldier Sir Edward Blayney.  He was succeeded by his son, the second Baron.  He was killed at the Battle of Benburb in 1646.  His younger son, the fourth Baron (who succeeded his elder brother), represented County Monaghan in the Irish House of Commons.  His elder son, the fifth Baron, was attainted by the Parliament of James II for supporting William of Orange.  He had no sons and was succeeded by his younger brother, the sixth Baron.  He was Governor of County Monaghan.  His son, the seventh Baron, was Lord Lieutenant of County Monaghan.  He was succeeded by his elder son, the eighth Baron.  He was a clergyman and served as Dean of Killaloe.  He had no surviving children and was succeeded by his younger brother, the ninth Baron.  He was a Lieutenant-General in the Army.  His younger son, the eleventh Baron (who succeeded his elder brother), was also a Lieutenant-General in the Army and fought in the Peninsular War.  Lord Blayney also represented the rotten borough of Old Sarum in Parliament.  His son, the twelfth Baron, sat as Conservative Member of Parliament for County Monaghan and was later an Irish Representative Peer from 1841 until his death.  On his death in 1874 the title became extinct.

Biddulph baronets: The Biddulph Baronetcy, of Westcombe in the County of Kent, is a title in the Baronetage of England.  It was created on 2 November 1664 for Theophilus Biddulph, of Westcombe Park, Greenwich, Kent, Member of Parliament for the City of London and Lichfield.  His son, Michael, the second Baronet, also represented Lichfield in the House of Commons.  This line of the family failed on the death of the second Baronet's son, Theophilus, the third Baronet, in 1743.  The late Baronet was succeeded by his first cousin once removed and namesake, Theophilus, the fourth Baronet.  He was the son of Edward Biddulph, elder son of Simon Biddulph, younger son of the first Baronet.  His grandson, Theophilus, the sixth Baronet, fought in the Battle of Waterloo and served as High Sheriff of Warwickshire in 1849.  He was succeeded by his son, Theophilus, the seventh Baronet.  He was a Deputy Lieutenant and Justice of the Peace for Warwickshire.  On the death of his son, Theophilus, the eighth Baronet, in 1948, the line of Edward, elder son of Simon Biddulph, younger son of the first Baronet, failed.  The late Baronet was succeeded by his fourth cousin once removed, Francis, the ninth Baronet.  He was the great-great-grandson of Walter Biddulph, younger son of Simon Biddulph.  As of 2008 the title is held by his grandson, Jack, the eleventh Baronet, who succeeded his father in 1986.  The three most recent baronets have resided in Australia.

Cunard baronets: The Cunard Baronetcy, of Bush Hill in Nova Scotia, was a title in the Baronetage of the United Kingdom.  It was created in 1859 for Samuel Cunard, the Canadian-born British shipping magnate.  He was succeeded in the baronetcy by his elder son Sir Edward, the second baronet.  Upon the second baronet's sudden death in 1869, he was succeeded by his eldest son, Sir Bache, the third baronet.  The third baronet married Maud Cunard, by whom he had one child, Nancy Cunard, and was succeeded in the title at his death in 1925 by his younger brother Sir Gordon, the fourth baronet.  The fourth baronet survived his brother by eight years; at his death in 1933, he was succeeded by his eldest son Sir Edward, the fifth baronet.  The fifth baronet was a lifelong bachelor, and at his death in 1962, the senior line of descent from the first baronet failed.  He was succeeded by a second cousin once removed, Sir Henry, who became the sixth baronet and was a great-great-grandson of the first baronet through his younger son William (1825-1906).  Also a bachelor, the sixth baronet was succeeded by his younger brother Sir Guy, the seventh and last baronet, in 1973.  He was likewise a bachelor and the title became extinct upon his death in 1989.

Baron Rokeby: Baron Rokeby, of Armagh in the County of Armagh, was a title in the Peerage of Ireland.  It was created in 1777 for The Most Rev. Dr Richard Robinson, Lord Primate of All Ireland and Lord Archbishop of Armagh, with remainder to his brothers and his father's second cousin Matthew Robinson and the heirs male of their bodies.  In 1785 he also succeeded his elder brother as 3rd Baronet according to a special remainder (see below).  Lord Rokeby never married and was succeeded in the barony and baronetcy according to the special remainders in the letters patent by his third cousin Matthew Robinson-Morris, the second Baron and fourth Baronet.  He was the son of Matthew Robinson (by his wife Elizabeth, daughter of Thomas Drake and his wife Sarah, daughter of Thomas Morris, of Mount Morris), son of Thomas Robinson, son of Sir Leonard Robinson, brother of William Robinson, great-grandfather of the first Robinson Baronet (see below) and the first Baron Rokeby.  The second Baron was an academic, politician and eccentric.  Born Matthew Robinson, he assumed by Royal licence the additional surname of Morris in 1746 on succeeding to the Mount Norris Estate through his mother.  He never married and was succeeded by his nephew Morris Robinson, the third Baron.  He was the elder son of Morris Robinson.

Georgiana Cavendish, Duchess of Devonshire: Georgiana Cavendish, Duchess of Devonshire ("née" Spencer; ; 7 June 1757 – 30 March 1806) was an English socialite, style icon, author, and activist.  Of noble birth from the House of Spencer, married into the House of Cavendish, she was the first wife of William Cavendish, 5th Duke of Devonshire, and the mother of the 6th Duke of Devonshire.

William Cavendish, Marquess of Hartington: Major William John Robert "Billy" Cavendish, Marquess of Hartington (10 December 1917 – 9 September 1944) was an English politician and soldier.  He was the elder son of Edward Cavendish, 10th Duke of Devonshire, and his wife, Lady Mary Gascoyne-Cecil.  He was the husband of Kathleen Kennedy, sister of the future U.S. President John F. Kennedy.

West Derbyshire by-election, 1944: The West Derbyshire by-election of 1944 was held on 17 February 1944.  The byelection was held due to the resignation of the incumbent Conservative MP, Henry Hunloke.  Hunloke was the brother-in-law of Edward Cavendish, 10th Duke of Devonshire, who had held the seat from 1923 until 1938, when he succeeded to his title and was replaced by Hunloke.  The seat had been held previously by Hunloke's father-in-law (1891-1908, before becoming the 9th Duke), and by the 9th Duke's brother-in-law, the future 6th Marquess of Lansdowne (1908-1918).

Supporting paragraphs: William Cavendish, Marquess of Hartington Kathleen Cavendish, Marchioness of Hartington
Answer: Kathleen Agnes Cavendish