I was not able to find this family in census records before or after 1870 either, but I did find the following marriages in Noxubee Co. MS. Don't know if they are the same George, Joseph, and Riley, but their ages in 1870 census makes me think they could be.
George Hopkins married Fannie Bracey 2 Sep 1877 Noxubee Co. MS
https://familysearch.org/ark:/61903/1:1:V287-5CK Joe Hopkins married Caroline Campbell 31 Jan 1878 Noxubee Co. MS
https://familysearch.org/ark:/61903/1:1:V28H-84JRiley Hopkins married Harriet Gilkey 29 Nov 1898 Noxubee Co. MS
https://familysearch.org/ark:/61903/1:1:V2Z8-BYCKen (not related)