William Sibbitts was from Somerset County, PA, as I recall, and was of German background - his father was also William Sibbitts. The original name was "Sibetz" I believe and was Anglized. They are a Pennsylvania Dutch family (German). "Sibbits" and "Sibbitts" are almost always German, but there are some who were from formerly German areas of Poland, Hungary, and other Slavic areas as well.
William Sibbitts served in the Indian Wars in the US Army after the Civil War, and was stationed briefly in Arizona before moving to Wolfpoint, Montana - he was quite prominent. There is a biography on him, as I recall, I shall try to look for it. I think he married a Native American, or his son married a Native American.
The name "Sibbitts" is German, the name "Sibbitt" is Scots or Scot-Irish, and to some extent English as well. The two families are completely different, however, there are both Sibbitts and Sibbit (or Sibbitt) in Montana now.