Marriage and children
On 2 June 1815, Petre married his first wife Frances Charlotte Bedingfeld (19 April 1796 – 29 January 1822 Thorndon Hall), daughter of Sir Richard Bedingfeld, 5th Baronet and his wife Charlotte Georgiana Jerningham. Her maternal grandparents were Sir William Jerningham, 6th Baronet and Frances Dillon. The senior Frances was a daughter of Henry Dillon, 11th Viscount Dillon and Lady Charlotte Lee. The senior Charlotte was a daughter of George Lee, 2nd Earl of Lichfield and Frances Hales. The eldest Frances was a daughter of Sir John Hales, 4th Baronet.
Frances, nicknamed "Fanny", became a letter writer of brilliant vividness. They had four children. Frances died in childbirth.
William Bernard Petre, 12th Baron Petre (1817–1884).
Hon. Mary Agness Petre (1818–1886), who married James Alexander Douglas.
Hon. Henry William Petre (1820–1889), a member of the New Zealand Legislative Council and father of Francis Petre.
Hon. Charlotte Elizabeth Petre (1822–1903), who married her first cousin, Capt. Charles Edward Petre, a son of Hon. Charles Berney Petre (a younger brother of the 11th Baron).
On 14 April 1823, Petre married his second wife Emma Agnes Howard (5 November 1803 – 10 February 1861 London). She was a daughter of Henry Howard of Corby Castle and his second wife Catherine Mary Neave. Her paternal grandparents were Philip Howard and Ann Witham. Philip was a son of Thomas Howard and Barbara Musgrave. Thomas was a son of William Howard and Jane Dalston. William was a son of Sir Francis Howard and Margeret Preston.
Sir Francis was a son of Lord William Howard and Elizabeth Dacre. His paternal grandparents were Thomas Howard, 4th Duke of Norfolk and his second wife Margaret Audley. His maternal grandparents were Thomas Dacre, 4th Baron Dacre and Elizabeth Leyburne.
Petre and Emma had eight children, including;
Hon. Frederick Charles Edmund Petre (1824–1906), who married Georgiana Musgrave, daughter of Rev. Sir Christopher Musgrave, 9th Baronet, in 1847
Hon. Agnes Catherine Louisa Petre (1826–1891), who married Charles Clifford, 8th Baron Clifford of Chudleigh.
Hon. Arthur Charles Augustus Petre (1827–1882), who married Lady Katherine Howard, a daughter of William Howard, 4th Earl of Wicklow.
Hon. Edmund George Petre (1829–1889), father of Francis Loraine Petre, the military historian.
Hon. Albert Henry Petre (1832–1917), who married Katherine Elsie Clark, a daughter of William Robinson Clark.