Your father must be related by one of John's Children. Here is a list of his children. He was married only to one woman and her name was Eliza J Hock (aka Hawk).
1.John Sisemore B. 1822 KYD. 29 Dec 1900 OK +Eliza J. Hock (aka Hawk) B. 1823 TN D. 1902 AR 2. George W. Sisemore B. 1845 AR D. 1922 AR Thomas Sisemore B. 1848 AR Isaiah Siemore B. 1850 AR William Van Sisemore B. 1852 AR D. 1889 AR Mary Sisemore B. 1854 AR Green G. Sisemore B. 1856 AR D. 1880 AR Marcus D. Sisemore B. 1859 AR D. 1942 NM These are the only children of John that I know of. Is Robert the son of one of these? Since my list is lacking in spousual inf and children of the marriage, it is a possibility that John is your ancestor. I have only limited info on Georges wife but I do know that he moved to Oklahoma after he married and passed away there in Oklahoma and is buried beside his wife Julia Anne Smith John, Eliza, Green G, William Van and two of his children and Marcus D.'s infant son is burried in Aurora Cemetery in Aurora, AR. I hope this helps you. Marcus D. is who I am named after.