1880 U.S. Census has this family listed as Robert and Sarah C. JOHNSON, not JOHNSTON. The children listed there were Robert age 8, William H. 6, John M.4, Mary E.3, Lorena 3mos. Also listed are the grandmother Elisabeth Johnson 78, Robert Sr.'s sister Sarah Johnson 59, and nephew Robert COOK 24, and nieces Jane COOK 21 and Elisabeth COOK 16. My grandfather David Hamilton WEIR lived with them from 1889 to 1892. He said that the JOHNSTON family consisted of 4 sons and 4 daughters and a total household of 15 people.
The FHL list their marriage in the IGI as 17 Jan 1871, Belmont, Ohio.