President Lady Helen Munro Ferguson by H. Walter Barnett (Hyde Park Corner SW), ca. 1914-1918
She was the daughter of Hariot Georgina Hamilton-Temple-Blackwood, Marchioness of Dufferin and Ava. She was born as Lady Hermione Catherine Helen Hamilton-Temple-Blackwood (1869–1960), and married Ronald Munro Ferguson, 1st Viscount Novar. Probably photographed while her husband was Governor-General of Australia from 1914 to 1920.
And these were her eldest siblings, see my other post here.
source: Imperial War Museums