Did you know you posted your message in Lewis County, Idaho? Not a big deal but it confused me. Or do you think Sarah or George was born there?
I found the 1880 Jackson, Greene Co., PA census where Walter was living with the Kitchel or Mitchel family and listed as adopted.
I looked for his parents in the 1870 Census but didn't find them.
I'd say your next step is to find Sarah and George's death information. I'm not familiar with searching in PA, however, I'd contact Greene County Pennsylvania Clerk or do a Google search for Greene County Pennsylvania Genealogy. That should bring up several web sites and hopefully one will have death info.
I figure Sarah and George were born around 1848 in Pennsylvania. They both must have died before 1880 Census in Greene Co.
Also look for a copy of Walter's birth certificate or where it's written in a ledger book.
The next thing is to post your query on the Greene County, PA Message Board. Successful Searching!