Marilyn, I don't know if you've found your McKinzie brothers' graves since your post; I've checked ancestry.com and on the Aldrich-Hix Family Tree, John H. McKinzie is shown bn Mar. 1847 in GA; son of James Henry and Sussana Givens McKinzie; mar. 1) Mary Trammel in 1869; was in Washington Co. AR 1860 census; mar. 2) Nancy Jane Mosely in 1896; and, in 1900 Fed. Census, was living in Township 5, Chickasha Nation...(Pontotoc County is in Chickasaw Nation);
So he could be buried around Ada. No death date shown on the FT; I did not find either John or George on Findagrave search, but they still could be in another cemetery listing in the area...go to Pontotoc Co. Cemeteries...many of the cemeteries have been canvassed and have a listing of names.
I'm no kin but know people who have lived in and around Ada, OK. Good luck.