Maud Hamerton was the niece of Whistler's model and mistress Maud Franklin, whose younger sister Margaret Kate Franklin (b. ca. 1858) married Albert Miles Hamerton on 5 February 1877), and died in Fulham, London, in 1886. Maud Franklin was deeply distressed by her sister's death, and very fond of her niece.
According to the UK census, Maud Taylor Hamerton (father A.M. Hamerton) was born in Dunedin, New Zealand ca 1881, but returned to the UK (she was listed at school in Chipping Norton, Hertfordshire in the 1891 census) By 1901 she was living in London with Lucy Edith Crossley, presumably Maud Franklin's elder sister Lucy Franklin, who was born in 1860 and died in 1902. It is not certain that she was married, any more than Whistler's mistress Maud Franklin although she likewise, in the 1881 census, gave her 'married' name, 'Mary M. Whistler'.
Maud Hamerton married George Edward Hayes Sheen on 23 April 1906 at St Paul's, Kensington. Maud Hayes-Sheen (or Maud Taylor Hayes-Sheen) died on 6 August 1950.
Another Maud, but not the right one, was Alice Maud Hamerton, father George Henry Hamerton, who was born in Kensington in July/August 1882 and married Alfred Charles Wright on 4 August 1901 in Marylebone.
The name might suggest a relationship to the artist and critic, Philip Gilbert Hamerton (1834-1894), but none of his children was called Maud.
Little Maud Hamerton y347 was painted by Whistler (the painting was listed among 'big pictures' in 1886 or 1887) but the picture has not been located.
UK census 1861 et seq; records, PRO, Kensington 1a 53 July-September 1882; The Correspondence of James McNeill Whistler, 1855-1903, edited by Margaret F. MacDonald, Patricia de Montfort and Nigel Thorp; including The Correspondence of Anna McNeill Whistler, 1855-1880, edited by Georgia Toutziari. Online edition, University of Glasgow, 2004.