William "Wild Bill" Keyser is my ancestor by way of John Jack his son, settling close to the mouth of the Big Sandy in Cabell Co. c. 1800.
While I would love to include Slammin Sammy Snead in my tree, descendant of Richard Snead b c 1765, I am in agreement with your discussions and research and find nothing that would identify Keziah wife of William as Richard's sister or even identify Keziah as a Snead. (her parents unknown).
Considering the close relationship between the Keysers and the Sneads in Hancock and Bath County, I believe if any such relationship described above was so, it would be well known.It appears they were nieghbors going west together. Considering that, I find it a little rare that their children never married each other, but that is probably just the way things are.