The 1870 census was your only hope of finding them earlier. The search engine does not locate them so......? Evidently Sam Brown expired prior to next available census in 1900? Robert Brown enumerated with the shortened version, Bob, of his given name. He remarried Lizzie Fitts, 4 Nov 1887 and apparently died prior to 1920. Lookup death certificates: Elmira Hollins (9 Jan 1873 - 1 Feb 1961) and Mary Brown Laws (17 May 1879 - 17 May 1959). Several of Robert's kid by Lizzie also have online death certificates. Not all marriage data is online and you may have to lookup info at County Courthouse.