I am a Midkiff descendant and most of the legal documents signed down through this line include relatives, either by blood or marriage. I don't know much about Jacob, though I've seen his name on documents and I don't know a lot about James either. I do know there were brothers John, Joseph and James. Most likely Jacob was a brother or cousin.
I would like to answer your question about whether or not Jacob or James were Davidson relatives but those are the two in the family for which I do not have a marriage record (Jacob and James). From the other family documents it sounds like Jacob most likely would have been an in-law but I have no proof.
These families were from Anglican, Quaker and Primitive Baptist backgrounds with the Midkiffs mostly being Primitive Baptist, their spouse's families mostly Quaker and other relatives mostly Anglican or Quaker. I know there should be Quaker records available if they could only be located.