On 30 Jun 1737 [his father] Alexander Stewart (age 39) and [his mother] Mary Cowan were married.

Before 1769 Robert Stewart 1st Marquess Londonderry (age 29) and Sarah Frances Seymour-Conway (age 21) were married. She the daughter of Francis Seymour-Conway 1st Marquess Hertford (age 50) and Isabella Fitzroy Countess Hertford (age 42). She a great x 2 granddaughter of King Charles II of England Scotland and Ireland.

On 10 Feb 1772 George Mason Villiers 2nd Earl Grandison (age 20) and [his former sister-in-law] Gertrude Seymour-Conway (age 21) were married. She the daughter of Francis Seymour-Conway 1st Marquess Hertford (age 53) and Isabella Fitzroy Countess Hertford (age 45). He the son of Alan Mason Viscount Grandison and Elizabeth Fitzgerald Villiers 1st Countess Grandison. She a great x 2 granddaughter of King Charles II of England Scotland and Ireland.

On 21 May 1775 Henry Fiennes Pelham-Clinton (age 24) and [his former sister-in-law] Frances Seymour-Conway were married. She the daughter of Francis Seymour-Conway 1st Marquess Hertford (age 56) and Isabella Fitzroy Countess Hertford (age 48). He the son of Henry Fiennes Pelham-Clinton 2nd Duke Newcastle-under-Lyme (age 55) and Catherine Pelham Duchess Newcastle under Lyne. She a great x 2 granddaughter of King Charles II of England Scotland and Ireland.

On 07 Jun 1775 Robert Stewart 1st Marquess Londonderry (age 35) and Frances Pratt Marchioness Londonderry (age 24) were married. She the daughter of Charles Pratt 1st Earl Camden (age 61) and Elizabeth Jeffreys Countess Camden (age 50).

On 25 Apr 1780 Walter Head aka James 1st Baronet and [his sister-in-law] Jane Pratt Lady James were married. She the daughter of Charles Pratt 1st Earl Camden (age 66) and Elizabeth Jeffreys Countess Camden.

In 1785 [his former brother-in-law] Hugh Seymour-Conway (age 25) and Anne Horatia Waldegrave (age 23) were married. She the daughter of James Waldegrave 2nd Earl Waldegrave and Maria Walpole Duchess Gloucester and Edinburgh (age 48). He the son of Francis Seymour-Conway 1st Marquess Hertford (age 66) and Isabella Fitzroy Countess Hertford. They were fourth cousins. He a great x 2 grandson of King Charles II of England Scotland and Ireland. She a great x 2 granddaughter of King James II of England Scotland and Ireland.

In 1794 [his son] Robert Stewart 2nd Marquess Londonderry (age 24) and [his daughter-in-law] Amelia Hobart (age 21) were married. She the daughter of John Hobart 2nd Earl Buckinghamshire and Caroline Conolly Countess Buckinghamshire. He the son of Robert Stewart 1st Marquess Londonderry (age 54) and Sarah Frances Seymour-Conway. He a great x 3 grandson of King Charles II of England Scotland and Ireland.

In 1795 [his former brother-in-law] George Seymour-Conway (age 31) and Isabella Hamilton were married. He the son of Francis Seymour-Conway 1st Marquess Hertford and Isabella Fitzroy Countess Hertford. He a great x 2 grandson of King Charles II of England Scotland and Ireland.

In 1804 [his son] Charles William Vane 3rd Marquess Londonderry (age 26) and [his daughter-in-law] Catherine Bligh (age 29) were married at St George's Church, Hanover Square. She the daughter of John Bligh 3rd Earl Darnley. He the son of Robert Stewart 1st Marquess Londonderry (age 64) and Frances Pratt Marchioness Londonderry (age 53).

Before 09 Feb 1810 [his son-in-law] Charles Fitzroy (age 45) and [his daughter] Frances Stewart (age 32) were married. She the daughter of Robert Stewart 1st Marquess Londonderry (age 70) and Frances Pratt Marchioness Londonderry (age 59). He the son of Augustus Henry Fitzroy 3rd Duke Grafton (age 74) and Anne Liddell Duchess Grafton. He a great x 3 grandson of King Charles II of England Scotland and Ireland.

On 29 Jun 1814 [his son-in-law] John James and [his daughter] Emily Jane Stewart Viscountess Hardinge (age 25) were married. She the daughter of Robert Stewart 1st Marquess Londonderry (age 74) and Frances Pratt Marchioness Londonderry (age 63). They were first cousins.

On 03 Apr 1819 [his son] Charles William Vane 3rd Marquess Londonderry (age 41) and [his daughter-in-law] Frances Vane Tempest Marchioness Londonderry (age 19) were married at Bruton Street. The difference in their ages was 22 years. He the son of Robert Stewart 1st Marquess Londonderry (age 79) and Frances Pratt Marchioness Londonderry (age 68).