Robert Adair and Lady Caroline Keppel had two daughters and one son. One of the daughters - Elizabeth Adair b. 1769 Middlesex, London, d. 2 Mar 1841 Shrivenham, England. married George Barrington 12 Feb 1788. They had son William Keppel Barrington b. 1 Oct 1793 Middlesex, London, England, d. 9 Feb 1867 Beckett House, Shrivenham, Berkshire, England. William Keppel Barrington m. Jane Eliz. Liddell and had dau, Charlotte Maria Barrington b. 29 Dec 1826 m. 30 April 1850 Thos. Geo. Lyon-Bowes, 12th Earl of Strathmore. Thos. Geo. Lyon-Bowes (whose wife was g-grand-dau of "Robin") was bro. of 13th Earl of Strathmore - Claude Bowes Lyon b. 21 July 1824 who was grandfather of present Queen Mother. Wonder if this holds any clues to your Sir John Sidney ?