The DNA tests proved only that her descendants have some Jeffersonian genes, which most probably is goes to Thomas' nephew/s.
This story was a political SMEAR JOB, which has lingered and sadly, now is taken as "fact".
Records of visitors to Monticello, do not support Jefferson’s nephew as the father of Sally Hemings children. However Thomas Jefferson was present at Monticello at around the time that Sally Hemings would have became pregnant with her children.