Here's a link that says she's some kind of nut.
http://books.guardian.co.uk/news/articles/0,6109,615448,00.html

Sickert, Walter Richard (1860-1942), German-born English painter, who painted urban life and genre scenes. He was a pupil of the American painter James Abbott McNeill Whistler. Later, influenced by the coolly analytical paintings of the French artist Edgar Degas, he painted realistic scenes of London theaters, pubs, music halls, and humble interiors. His enthusiasm for his rough, sometimes sordid, subject matter gave many of his pictures verve and excitement.



"Sickert, Walter Richard," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation. All rights reserved.

A German site says Joseph was "uneheliche" son of Walter. Mother's name not given.