 He was the eldest of seven children born to Adrian Georg Iselin (1818–1905) and Eleanora (née O'Donnell) Iselin (1821–1897).His younger siblings included William Emil Iselin; Eleanora Iselin (wife of DeLancey Astor Kane); Columbus O'Donnell Iselin (husband of Edith Colford Jones); Charles Oliver Iselin; Georgine Iselin, who was made a Papal Countess in 1912 and did not marry; Emilie Eleanora Iselin (wife of John George Beresford, a grandson of Henry Beresford, 2nd Marquess of Waterford).in 1898.