Deborah Delano Haden, née Whistler, was the daughter of George Washington Whistler and his first wife Mary Roberdeau Swift. She had two brothers, George William and Joseph Swift, and also five half-brothers, James Abbott, William McNeill, Kirk Boott, Charles Donald and John Bouttatz, her father having married a second wife Anna Matilda McNeill in 1831 following Mary's death in 1827. On 16 October 1847, Deborah married Francis Seymour Haden, a surgeon, collector and etcher; JW was groomsman at the wedding. Deborah and Seymour had four children, Annie Harriet (later Mrs Charles Thynne), Francis Seymour, Arthur and Harry.
In November 1858, JW stayed with Deborah (whom he affectionately referred to as 'Debo' or 'Sis') and her family at 62 Sloane Street, London. There he painted his half-sister along with her daughter Annie in At the Piano (YMSM 24). JW also depicted the family in Reading by Lamplight (K.32), The Music-Room (K.33).
In April 1867 JW quarrelled with Seymour Haden and pushed him through a plate-glass window in Paris. The two never spoke again, and it was a while before Deborah could communicate with her brother.
Harmony in Blue: The Duet (YMSM 196) was given to her by JW as a present, although J. J. Cowan persuaded JW to swap it for a different work so that he could purchase it. Deborah was musical, having inherited her father's talent, and played the piano.
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.