I think Martha is the aunt of Joseph Jefferson Sledge. Martha's father, Henry M. Sledge, is listed on the 1840 Monroe County census. There are 3 boys and 1 girl listed in the household. The other boys and the girl are listed on the 1850 census with Henry's widow, Nancy (Fuller). The one boy age 5 - 9 must be John. John Sledge and Sarah Duncan are also listed as the parents of John Sledge, who married Maggie Winder/Wiseman. Henry and Nancy's youngest daughter, Sarah Sledge, also married a Duncan (Henry C.) I do not think he was a brother to Sarah Elizabeth Duncan because he had two sisters named Sarah and Elizabeth, but perhaps they were cousins.