“Yeah, he probably did have children with his sister-in-law, Sally Hemmings. She was his wife’s half sister and was raised in their household, so though her status was slave, she was part of the household and one of several unacknowledged family members.”
I’ve read compelling evidence that it was a brother who would often visit, and I couldn’t care less either way, but I think they’re in a rush to judgment as far as Thomas himself is concerned. The “half-sister” relationship you mention only makes me more skeptical; that makes him really freaky if he did sire children with her.
Jefferson’s wife Martha died in 1782 when Sally Hemmings was 9 years old.