Hmm …

… George Washington’s Biracial Family Is Getting New Recognition | Smart News | Smithsonian. (Hat tip, Rus Bowden.)

OK, so this is Washington's adopted son. So he isn't a blood relative of the first president. Then "there is plenty of circumstantial evidence to suggest that Parke Custis had an unusually close relationship with Branham and her daughter." But until we have a genetic test, we can't know that for sure. I think the test would almost certainly affirm the matter and ought to be done. Because history is supposed to be about facts, not hearsay.