People need to cut Jefferson a little slack here. He was still young and virile when his wife died at such a young age. It would have been quite natural for Sally, 3/4th white and stunningly beautiful by most historical accounts, to step in to fill a gaping need in Jefferson's life.
Sally could have left him during the years they were in Paris together when he was living as ambassador to France since slavery had been abolished by the revolution. The fact that she did not indicated she cherished the relationship.
Prudes today can hop on their soapbox and claim he should have formally married her. At the time, such a marriage would have, in fact, been illegal.
Supposedly Jefferson promised his wife just before her death that he would not remarry. He easily could have found a white woman of his social class to agree to marry him.
The DNA results make it likely that Jefferson fathered some of Sally's children but it is not 100% certain (several close male relatives with the Jefferson Y-chromosome are possibilities). One of her children may have been fathered by one of Jefferson's nephews (two sons of his sister, surname Carr, were long suspected, and often mentioned pre-1998 by those defending Jefferson's reputation).
Sally was the half-sister of Jefferson's wife.
Sally was around 50 when Jefferson died. He may have believed she was better off not being legally free and that there was no danger that she would be given onerous work to do.
TJ promised his dying wife that he would not remarry, apparently the Evil Step Mother stereotype had some influence. I have no doubt that his relationship with Sally was one of love on both parties.