I am of Rufus Alexander Johnston's direct line.
When my grandmother (Patricia Ann Johnston) died we were given two binders full of jumbled notes. I am still trying to make heads or tails of them. They seem to point back to a Wiley Johnston that was born around 1786 in North Carolina.
I can tell you, however, that in the notes I did find mention that George A Johnston was the son of John Johnston and that he did marry his cousin Sarah Elizabeth Johnston who was the daughter of Harrison Johnston.
I have both John and Harrison as being the sons of Wiley Johnston and Harriet C. (Brown maybe).
Hope this helps in some small way.