Isiah Foster first shows up in the 1920 census of Pemiscot county, Missouri: Household Gender Age HeadSamuel T FosterM 33y born Tennessee SpouseAlpha FosterF 25y born Tennessee ChildArless A FosterM 7y born Missouri ChildIsiah FosterM 6y born Missouri ChildEddie FosterM 4y3m born Missouti George BlankenshipM 20y born Tennessee The 1920 census record is here:
By 1930 the family is living at Little River, Pemiscot, Missouri: Household Gender Age HeadTaylor FosterM 43 SpouseAlpha FosterF 38 ChildUles FosterM 17 ChildIsaiah FosterM 16 ChildEddie FosterM 14 The 1930 census record is here: https://familysearch.org/pal:/MM9.1.1/XHVY-5TB
Alpha Blankenship first shows up in the 1900 census of Civil District 12, Weakley, Tennessee: Household Gender Age HeadPles C BlankenshipMborn feb 1863 Tennessee SpouseAmanda C BlankenshipFborn Oct 1872 Tennessee, married 9 years, mother of 5 children ChildAlbert H BlankenshipMborn May 1892 Tennessee ChildAlpha J BlankenshipFborn Nov 1893 Tennessee ChildAlfred C BlankenshipMborn Nov 1895 Tennessee ChildAlvin R BlankenshipMborn Nov 1897 Tennessee ChildCarrie K BlankenshipFborn Mar 1900 Tennessee ChildJames L BlankenshipMborn Jul 1885 Tennessee Mother-in-law Amanda K RiggsFborn Jul 1848 Tennessee Sister-in-law Emma J RiggsF born Feb 1878 Tennessee The 1900 census record is here: https://familysearch.org/pal:/MM9.1.1/MSZK-17S
I suspect that Amanda Riggs Blankenship is a second wife and that James L Blankenship was the son of the first wife.
In the 1910 census of Hayti, Pemiscot, Missouri, Taylor Foster is living with the Blankenship family, but he and Alpha have not yet married: Household Gender Age HeadC BlankenshipM 48y SpouseMaudy C BlankenshipF 38y ChildAlbert BlankenshipM 17y ChildAlpha BlankenshipF 16y ChildAlfred BlankenshipM 14y ChildClara BlankenshipF 10y ChildMarvin BlankenshipM 8y ChildHubert BlankenshipM 5y Taylor FosterM 21y The 1910 census is here: https://familysearch.org/pal:/MM9.1.1/M2BK-7T5
There is a 6 August 1891 marriage for Amanda A. Riggs and D. Blankenship at Decatur county, Tennessee.The marriage record is here: https://familysearch.org/pal:/MM9.1.1/XD9N-V3K
Pleasant C Blankenship shows up in the 1870 census of 15th Civil District, Henderson, Tennessee, as the youngest son of Sarah Blankenship: Household Gender Age Sarah J BlankenshipF 32y Malinda E BlankenshipF 13y Mary E BlankenshipF 11y Nancy J BlankenshipF 9y Pleasant C BlankenshipM 7y The 1870 census record is here: https://familysearch.org/pal:/MM9.1.1/MDDW-QSX