I also am a descendant of Douglass Carroll; and his son David who lived in Henry County, Georgia, dying there in 1856. Studying this family for a little over two years now, I can confirm that Grief Carroll was a brother to Douglass Carroll, and can also confirm that Sterling Carroll was some relation to Douglass, but whether he was a brother or an uncle, I don't know.
In your message you mentioned that Daniel Carroll might be a brother to both Douglass and Grief. Is that information confirmed by any document or records? Or how did you learn it?