From National Gallery of Art, Washingon:
Degas' family was relatively affluent, so he did not have to rely entirely on sales of his work for financial support. He was thus free to experiment and choose his own subjects; almost all of his portraits depict relatives or friends. He was also able to delay finishing paintings, reworking them until they met his exacting standards. Many times Degas retrieved works he had already delivered so that he could perfect them. Some he never completed.
This unfinished portrait of Degas' sister and her Neapolitan husband is one such example. (The painting was in his studio at the time of his death.) Notice how Thérèse's dress and shawl are undefined masses of color. There, Degas has scraped and rubbed the paint off the canvas. The dark lines indicate changes he intended but never made. The faces, by contrast, are carefully finished, detailed and expressive. Degas hoped to capture his sitters, he said, in "familiar and typical attitudes."