I am no fan of Jefferson but have no doubt that his relationship with Sally was one of love. She was his wife’s half sister and he had promised Martha on her deathbed that he would not remarry. Sally must have even looked like Martha.
As far as I know Jefferson had no other affairs after Martha died.
There is a possible sexual affair and certainly a flirtatious affair with Maria Cosway, a married English woman whose husband was apparently quite forbearing. It is simply not known whether that relationship was consummated but it appears to have been over just before the arrival of 14-year-old Sally Hemings in Paris. Certainly, Jefferson was quite smitten with Cosway and she evidently with him until the relationship cooled.