Annette Gordon-Reed discusses a tricky question in the latest American Heritage magazine (vol. 58 no. 5, Fall 2008): "Did Sally Hemings and Thomas Jefferson Love Each Other? A Historian Tackles One of American History's Thorniest Questions". Don't worry, I won't give away the ending, but Gordon-Reed brings up some intriguing facts and makes some thought-provoking observations.
Sally Hemings, whose given name was probably Sarah, was the daughter of Elizabeth (Betty) Hemings. According to her son, Madison Hemings, her father was Thomas Jefferson's father-in-law John Wayles. There are no known portraits of her.