Before 1704 [his father] Brabazon Ponsonby 1st Earl Bessborough (age 25) and [his mother] Sarah Margetson (age 25) were married. They were second cousins.

On 05 Jul 1739 William Ponsonby 2nd Earl Bessborough (age 35) and Caroline Cavendish Countess Bessborough (age 20) were married. His younger brother John Ponsonby (age 26) would marry her younger sister Elizabeth Cavendish (age 13) four years later. She the daughter of William Cavendish 3rd Duke Devonshire (age 40) and Catherine Hoskyns Duchess Devonshire (age 40). He the son of Brabazon Ponsonby 1st Earl Bessborough (age 60) and Sarah Margetson.

In 1743 [his brother] John Ponsonby (age 29) and [his sister-in-law] Elizabeth Cavendish (age 17) were married. His elder brother William Ponsonby 2nd Earl Bessborough (age 39) had married her elder sister [his wife] Caroline Cavendish Countess Bessborough (age 23) four years before. She the daughter of William Cavendish 3rd Duke Devonshire (age 44) and Catherine Hoskyns Duchess Devonshire (age 44). He the son of Brabazon Ponsonby 1st Earl Bessborough (age 64) and Sarah Margetson.

On 12 May 1748 Horatio Walpole 1st Earl Orford (age 24) and [his sister-in-law] Rachel Cavendish (age 20) were married. She the daughter of William Cavendish 3rd Duke Devonshire (age 49) and Catherine Hoskyns Duchess Devonshire (age 49).

On 04 May 1763 [his son-in-law] Aubrey Beauclerk 5th Duke St Albans (age 22) and [his daughter] Catherine Ponsonby Duchess St Albans (age 20) were married. She the daughter of William Ponsonby 2nd Earl Bessborough (age 59) and Caroline Cavendish Countess Bessborough. He a great grandson of King Charles II of England Scotland and Ireland.

In 1770 [his son-in-law] William Fitzwilliam 4th and 2nd Earl Fitzwilliam (age 21) and [his daughter] Charlotte Ponsonby Countess Fitzwilliam were married. She the daughter of William Ponsonby 2nd Earl Bessborough (age 66) and Caroline Cavendish Countess Bessborough. He the son of William Fitzwilliam 1st and 3rd Earl Fitzwilliam and Anne Watson Wentworth Countess Fitzwilliam.

On 27 Nov 1780 [his son] Frederick Ponsonby 3rd Earl Bessborough (age 22) and [his daughter-in-law] Henrietta Frances Spencer Countess Bessborough (age 19) were married. She by marriage Countess Bessborough. She the daughter of John Spencer 1st Earl Spencer (age 45) and Margaret Georgiana Poyntz Countess Spencer (age 43). He the son of William Ponsonby 2nd Earl Bessborough (age 76) and Caroline Cavendish Countess Bessborough.