Per the genealogy at
http://heirsofeurope.blogspot.com/search/label/Aga%20Khan, the late Mr Croker Poole's mother, Jean Margaret Balfour Watson (number 11 on the ascendancy list), was the great-granddaughter of Mackintosh BALFOUR (9 III 1825 - 7 VI 1884; number 94 on the list), who was himself a son of the Rev Lewis BALFOUR (30 VIII 1777 - 24 IV 1860) and his wife (married 26 IV 1808), Henrietta Scott SMITH (1787 - 13 III 1844). This Balfour family seems to have last appeared in Burke's Landed Gentry in the 1906 edition - see the article on Balfour-Melville of Mount Melville therein at
https://ukga.org/browse.php?action=ViewRec&DB=33&bookID=231&page=blg190601159.
Mackintosh Balfour's sister, Margaret Isabella BALFOUR (1829 - 1897), was married in 1848 to the lighthouse engineer, Thomas STEVENSON (22 VII 1818 - 8 V 1887), and their son was Robert Lewis Balfour STEVENSON (13 XI 1850 - 3 XII 1894), who, having changed "Lewis" to "Louis", and dropped Balfour, is better known as Robert Louis STEVENSON, the author of Treasure Island, among his other works. R L Stevenson was therefore a first cousin three times removed upwards of the late Mr Croker Poole.