Nancy Cash, originally from Kentucky but they came to Missouri later
She had a sister name Louisa Cash who married a Pettus, and her brother, I forget which one married a Adams. And maybe the Adams were somehow kin, but that would not make John his uncle.
The other option is John Roan's first wife, who was Catherine, and I think she was a Spalding but not positive. They had two son's together one being Allen Roan who was mentioned in the book as well, as being someone who traded Sam a horse and then Sam was accused of stealing it. Along with Allen was a Cooper, and we have Coopers in the family as you can see by my last name. Also Allen ws know for taking horses when he was a Union soldier.
Allen had a brother John Roan who was a Confederate soldier . But I did not read anything about him
One of the Roan girls married a Fremont Cooper but not sure it is the same Cooper mentioned in the horse trade as I cannot remember now been long time since I read it.

Yeah, I am kinda thinking he was probably not a REAL uncle of Sam's , but my husband says he remembers his mom telling him he was kin to the notorious Sam Hilderbrand. But he said she never explained how.