I can't answer your question about the parents of John Alexander but I would guess that he was related to Samuel Alexander b. 1759 in Chester County, Pennsylvania. However, Samuel grew up and lived his life in Burke County, North Carolina. He was the son of William and Elizabeth [-?-] Alexander.
Samuel Alexander m. Jean Penland in 1797 - she was the daughter of Robert and Elizabeth (Brank) Penland. The only sibling of this Samuel that I have notes on was Noble Alexander. There were undoubtedly others.
This information is from "Revolutionary War Soldiers of Western North Carolina: Burke County" volume one, by Emmett R. White. There is no specific information about your John.