We also have two manuscript letters of Civil War soldiers who noted that they visited Mount Vernon: Letter to Maria, by Andrew Bowers (101st New York Infantry, Company C), May 25, 1862; Letters of Hiel P. Clark (3rd Michigan Infantry, Company D), 18961-1869.
I recently read that Mount Vernon Ladies Association and the National Park Services, which run Mount Vernon, are finally acknowledging a truth they have strongly suspected or known for awhile, but chose not to publicly admit, that Martha's grandson (not George's) Parke Custis, had a white family and also had children with two slave women, Arianna "Airy" Carter and Caroline Branham. Parke fathered at least two girls with Airy, Maria Carter, who later married another slave at the house, which was a rare honor. Parke freed her and her two sons in 1826, leaving then 17 acres. Her son, John, was educated and serve in elective offices, and the other, William, was a chief messenger of the Dept. of the Interior and pushed for public high schools for African Americans in D.C. He also had two sons with Caroline Branham. Read more about this here http://www.history.com/news/organizations-acknowledge-george-washingtons-biracial-family-tree