She was one of the several children of Amy Leslie Forster (1852-1904) and Matthew Forster (1826-1881), a barrister, of Frere, Forster and Frere, solicitors.
Whistler was friendly with the Forster family, who lived at 37 Ennismore Gardens in Knightsbridge. The birth certificate records her middle name as Isabella, and in the 1881 UK census it is summarised as 'Y' but in the 1891 census it was given as Mabel.
Yvonne married Henry Dyson (1853-1904) and had two children, Harry and Maud.
About 1881/1884 Whistler drew Portrait of Miss Yvonne Forster m0838).
UK census 1861, 1881, 1891, birth and death records (Ancestry.uk); Young, Andrew McLaren, Margaret F. MacDonald, Robin Spencer and Hamish Miles, The Paintings of James McNeill Whistler, New Haven and London, 1980; MacDonald, Margaret F., James McNeill Whistler. Drawings, Pastels and Watercolours. A Catalogue Raisonné, New Haven and London, 1995.
'Forster Family Tree, Ancestry.com.