She was the daughter of John Alger Worth, a shipbuilder, and his wife Mary Anne, who by 1861 were living in Hampstead, London, and had three sons and three daughters.
Her husband, Matthew Forster (1826-1881), was a barrister, of Frere, Forster and Frere, solicitors. Their daughter, Yvonne Mabel (b. 21 October 1877), married Henry Dyson; Francis L.M. was born ca 1874 and Evelyn A.E., ca 1876. They had two more daughters, Maude Elizabeth and Nina Dorothy.
Whistler was friendly with the Forster family, who lived at 37 Ennismore Gardens in Knightsbridge. In a letter to his sister-in-law Nellie, dated 16 December 1881, Whistler's mentioned that he was working on a portrait of Amy Forster (GUW #06695). Her portrait was still in his Tite Street studio in 1884 when the artists Walter Sickert and J. E. Blanche saw it there. It was apparently a 'black on black' portrait, but nothing further is known of it (Portrait of Mrs Forster y234).
About 1881/1884 Whistler drew a picture of Amy Forster's daughter (Portrait of Miss Yvonne Forster m0838).
Mrs Forster was among those invited to the private view of the Royal Society of British Artists 1887-88 winter exhibition (GUW #13403). In 1892 she was also invited to the private view of Whistler's Goupil Gallery exhibition, Nocturnes, Marines and Chevalet Pieces (GUW #08449).
In 1891 the widowed Mrs Forster was living in Cadogan Square, Chelsea.
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.