Brothers from left to right: Henry Forbes (b. 1882) and Gerrit Forbes (1880-ca. 1965)
Image taken from Dovercards, however documentation and dating from The Smithsonian Catalogue of Works
Ormond, Richard and Elaine Kilmurray, "John Singer Sargent: complete paintings; volume 1, The early portraits," New Haven, CT: Yale University Press, 1998, no. 196.
McKibbin, David, "Sargent's Boston," Boston: Museum of Fine Arts, 1956.
Ormond, Richard and Elaine Kilmurray, "John Singer Sargent: complete paintings; volume 1, The early portraits," New Haven, CT: Yale University Press, 1998, pg. 201.