Mistress indicates consent to me, or at least the possibility of consent. A less confusing statement could be"Hemings, a slave, was the half-sister of Jefferson's wife who, after Martha Jefferson died, bore six of Jefferson's children." Abusive comment hidden. (Show it anyway.)
A less confusing statement could be
"Hemings, a slave, was the half-sister of Jefferson's wife who, after Martha Jefferson died, bore six of Jefferson's children."