Hi Barbara, I think Revolutionary Robert Sidney Adair was the son of Sheriff Robert Adair.
Milliner Key Letcher, Robert Adairs M-I-L, was the daughter of Henry Key of Augusta Co. You can refer to Gone to Georgia-Jackson and Gwinnett Counties and Their Neighbors in western Migration Annotated listing of the 1820 census in Gwinnett and Jackson counties.Page 168 has write up on Adairs in the area. Robert Sidney Adair is listed in both references. Excerpt from p 168 under ID 887 Joseph A. Adair--------- "Robert Sidney Adair married in 1774 Elizabeth (Posey) the widow Huffman and moved from Virginia to South Carolina.They had a son Robert S. Adair, born c1783, who married Martha Lefevre, daughter of Milliner (Key) Letcher Lefevre. James Letcher married Milly Key, Jan 6, 1771, in Amherst County, VA., with the consent of her father, Henry Key.The Adairs and Keys were associated as early as William Adair's 1763 Augusta County (VA) will when he named his daughter Sarah Keys." I don't know of any Harbison connections. Ed