John Demmon did have a brother named Charles. The Charles Demmon who married Carrie was probably that same brother.I don't know for sure because I have never searched for Charles.I can tell you that Eliza Van Patten was also a school teacher in Chicago. She met John Demmon there and they were married 12/26/1865 in Chicago.They owned lots of land in Carroll and Whiteside counties.
John Demmon was from Hartford Vermont.There is quite a bit of information on the Demmon family and there is a cemetery there where most of them are buried.
Eliza attended a secondary school in Pompey called Pompey Academy.Most likely, her siblings went there as well.
It also seems that many of the Van Patten and Demmon siblings were in Illinois at some point after John and Eliza settled there.
There was a Demmon family in Cayuga and Onondaga family...which is not so distantly related to the Vermont Demmons.I have often seen the NY family listed as Eliza's husband's family in many genealogies...which is not the case.
Let me know if you need more info on the demmons or Van Pattens