Your question is one that has provoked a lot of research and speculation for at least a century, including my own efforts for the past 36 years, but no one has succeeded in establishing the identity of Stephen's parents, so far as I am aware. There seem to be two possibilities: (1) he was the son of Thomas Pettus II of Littletown plantation, James City County, Virginia, and his first wife whose name is not known, or (2) he was the son of an earlier Stephen Pettus who settled in Blisland Parish, New Kent County, Virginia, in the 1660s. Mary Dabney was the daughter of Capt. George Dabney of King William County, Virginia (see his will--1729); however, some researchers have concluded that Stephen married one of Mary's sisters. The only evidence I have is the will, but it is a bit ambiguous and has a tear exactly where Stephen's name seems to be written. I will have a full discussion of the problem with my opinion on the subject in my forthcoming book, which I hope to publish by the end of the year.
As for the lines of descent from Stephen, the names of his sons, Stephen, Jr., Dabney, John, and George, are well-established, though birth dates are lacking. He may have had daughters, too, including Ann Phillips, but the evidence concerning them is not so strong as for the sons. I have traced the lines of descent from the various children for as many generations as they lived in Virginia and even a generation or two of descendants who migrated to other parts of the country.