These Hows are treated in BLG 1969 under 'How of Hillhouse'; the head of the family at that time, William Sawbridge How (of Hillhouse, Kirknewton, Midlothian, who m. 1947, Ludivina Frances, only dau. of Sir William Frederick Stuart-Menteth, 5th Bt, and had issue two sons and a daughter), was Robert Boothby How's elder brother, both being sons of William Fitzherbert How, himself elder son of John Herbert How (1855-1930) Maj. 3rd Bn Devon Regt, J.P. Bideford, Devon, by his wife Madeline Ellen, 3rd dau. of Lt-Col Robert
Tod Boothby, of St Andrews, Fife (whose grandson Robert John Graham Boothby, the Conservative politician, was cr. Baron Boothby). William Fitzherbert How married 1914, Silvia Bertha, dau. of Edward Henry Bridgman Sawbridge, J.P., Capt. South Wales Borderers.
Robert Boothby How (1914-1990) was educ. Trin. Coll. Cantab. (B.A.), Maj. in the Black Watch, according to Burke's Peerage 2003 married 1942 (div. 1953) Virginia Ruth Primrose, dau. of Oliver Hughes-Onslow (possibly of a landed family? It rings a bell, anyway) and had a son, Denzil Robert Onslow b. 1944, and two daughters, Primrose Jean Onslow b. 1947, and Carolyn Jane Onslow b. 1950. Presumably (I can't view the relevant part online) he married again, and Sally-Anne is a daughter of a subsequent marriage? A company director record for a Sally Anne Dalrymple Hamilton, of Ayrshire, gives her birth year as 1954, corroborating this.
Everard John Boothby How was R. B. How's uncle, being younger brother of William Fitzherbert How.