If George Riley Stilwell's natural mother married Burnt (sic?) then he was not adopted.
In 1900, George was enumerated with Darwin (Rob states David - the image of the census record that I viewed was difficult to read) and Ellen Stilwell in Stewart, York County, NE.
In 1910, George was living with his mother, Ellen, his stepfather, Burnt (sic) and a half brother and sister in Ansley, Custer County, NE.
His World War I draft card signed on September 12, 1918 indicates that he was living in Ansley, Custer County, NE.He was working on his stepfather's farm and he listed his mother as his nearest relative.
In 1920, he was living with his wife, Ollie (born NE) in Ansley, Custer County, NE.
In 1930, he was living in Whitman, Grant County. Nebraska.He was a widower at that time.
In all of the records mentioned above his name was recorded as George Stilwell.
His wife, Ollie, is buried in Ansley Cemetery.According to her tombstone she was born January 1, 1902 and died June 14, 1928.He tombstone inscription reads "Mother".
If George was adopted from a Sawyer family, his father was most likely George Sawyer who was living with his wife and two children (a son 14 years of age and a daughter 10 years of age) in York County. George was born in Vermont and was renting a farm.He may have been the brother of Ellen Stilwell Burnt (sic)This is pure speculation on my part.