I find in the Scott County, Arkansas Marriage Book A 1882-1890:
Mankin, R. S. 21 of Boles to Miss Viney Sumpter 17 of Boles by G. W. Sumpter J. P. on 20 December 1887, Page 375.
If you contact the Scott County Historical Society, there is a lady that for a reasonable fee will copy this record at the courthouse for you.
In the Scott County Cemetery book I find: Mankin, Jasper Lafate 5 Dec 1900 - 29 Jul 1903 s/o R.S. & Vinie. There is also: Fannie, Docia, Fanny, G. W., John, Lydia, Lydia Jane, Marta, Mary Frances, P. E., R. S., Vinie, William, and William. Note: some of those are not buried in Scott County - only mentioned as relatives of the deceased who is buried there.
1900 Scott County Census ED 111 Hickman Townshiip *262/376 Mankins, Robert S.HdApr 186733M/12TX MO MO VinieWfMay 1872285/5MO MO MO FrancisDFeb 189010AR TX MO MaryDMay 18937AR TX MO Robert M.SApr 18946AR TX MO SarahDMay 18955AR TX MO JamesSMay 18982AR TX MO
Then another family with slightly different info in Mountain Township: (note that on the children the father's birthplace is underlined indicating an error):
#47/53 Mankins, Robert S.HdMay 186832M/12AR TN TN MelvinaWfJan 1870305/5 MO MO MO Martha F.DMay 188812AR MO MO Mary A.DFeb 18919AR MO MO William R.SAug 18936AR MO MO Sarah J.DJul 18963AR MO MO James E.SSep 18981AR MO MO
There are these other Mankins in 1900: George W.; John; John A; William and William C. There are these Sumpter/Sumters: James D. Sumpter; Elsby A. Sumter; George Sumter; and Maston N. Sumter
By 1910, all Mankins are gone with the exception of W.C. and William.
Hope this helps. Really curious about the two Robert S. Mankins...