I also descend from this John Abney. R.R.Abney, Jr.has published monthly newsletters in the past.I have all of them and he in his letters answers this question better than I could.The letters were on the internet.I am not sure if they still are.But I would be happy to send copies of the letters if they are not available on the internet anymore.
This paragraph is from an e-mail that I received from him.
"I believe that Pvt. William Abney of the Am. Rev. War (who m. the Graves sisters) was not a son of Dr. Abraham Abney. I believe, again by deduction that he was the William Abney who m. in Amherst (like the other 4 siblings), and was the surety for Elizabeth, his sisterTherefore, I believe the William Abney of 1789 was Pvt. William Abney, grandson of Paul Abney (bapt.1699), and brother of Jonathan "John" Abney who m. Martha Patsy Puckett.Therefore, since we can prove that Paul (bapt.1699) was a son of Dannet Abney, we have this: Dannet m. Mary (Lee),Paul Abney (bapt. 1699) m. NN., nn(Hensell calls William))Pvt. Wm m. Graves sisters, Jno m. Martha Puckett, Elizabeth m.Tandy Holman, Mary m. John Brown, and Lucy m. Jesse King.There is too much to type here but I will be glad to help you with other material and letters and will get R.R.Abney's e-mail address to you soon.Hope this helps.According to his info, John would be a brother to your William. This does not answer yourquestion about the father of William and John.This is covered in one of the papers R.R.Abney has written.