Henrietta G. Cullen appears to have been the sister of Katharine, wife of the artist G. H. Boughton. Katharine was born in Walsall in 1845 and Henrietta in Kensington in 1847. The census lists Henrietta as the 'sister' of the head of the household, G. H. Boughton, but given that she was unmarried, and named Cullen, this seems unlikely. She was living with the Boughtons at West House, Campden Road, London, in 1881. Henrietta knew JW in the 1870s, and possibly acted as chaperone to her niece 'Flossie' Boughton.
UK census 1881 (accessed 2004.03) from http://www.familysearch.org