I think this is mixed up.My sources show that the Elizabeth Fenley (born 26 May 1720) who married Thomas Athey (born c 1695) was the daughter, NOT the sister, of Charles Fenley (who married Elizabeth Harris).
Charles Fenley may indeed have had a sister named Elizabeth, but I believe it was his daughter Elizabeth who married Thomas Athey.(BTW, Charles Fenley and Elizabeth Harris were married 11 Apr 1711 in Prince Georges, MD and had 10 children of which I am aware [per "Colonial Settlers of Prince George's County, Maryland", page 9, ref. King George's Parish Records, pp. 283, 290]).