Thank you, Darwin Kampman, for the reply to my quest for connections. Yes, there is an Isaac Nellis who was son of my great-great-great grandfather (John Nelles)......and another Isaac who was the son of my great-great grandfather (Charles Nelles). I think you're right about my great-great grandfather. He was born 25 July 1806/7 in Oneida, Haldimand Cop. Ontario, and died in 1879 in South Dakota. Married to Lavina Young. So, Isaac was the brother of my great grandfather, Walter Nelles.
This about all I know about them. Do you have any more than that, e.g. migration to SD, occupations, life in general?