Here are your census listings for Hoosier. I checked other indexes for the county but found no other mention of the name except in the cemetery book, (I have included the Hoosier names for you). In it we see that Harvey died in 1926 and Elizabeth in 1919. The state was issuing death certificates by 1917, so you're in luck. You should be able to apply for death certificates from the state of Illinois for both of them, and that will give you (hopefully) their parents' names. FIRST, you will need to order a county death certificate ($5) from the Brown County Clerk in order to get the full date of death for submission to the state.
I strongly suggest you get death certificates for your mother's parents as the first step in this search. Your grandfather Hoosier's death certificate will give his parents' full names.
I must admit that I find it odd that the family doesn't appear in the 1880 census, and their are no marriages in Brown County for the Hoosier surname, and yet they are buried in Versailles. I can see why you are puzzled by them.
Best of luck,
1870 Fed Census, Versailles twp, Brown Co, IL 131/131 (3 Aug):
Harvey Hoosier, 25, farmer, b. TN
Elizabeth Hoosier, 24, b. IL
Fanny B. Hoosier 6, b. IL
Willie K. Hoosier 3, b. IL
Harvey Hoosier 1, b. IL
1870 Fed Census, Versailles twp, Brown Co, IL 166/166 (4 Aug):
John C. Murray 38, farmer, b. IN
Julia Murray 36, b. IN
Joseph Murray 12, b. IL
Ellen Hoosier 11, b. IL
Cemeteries of Brown Co, IL, p. 387 (Westside Cem, Versailles):
John R. Hoosier (1879-1931) Son of Harvey & Elizabeth
Eldon Vaughn Hoosier (1906-1943) Son of John R. Hoosier
Harvey Hoosier (1844-1926)
Elizabeth Hoosier, wife of Harvey (1845-1919)
Alva Hoosier (1870-1946) Son of Harvey & Elizabeth
Pearl Hoosier (1881-1926) Wife of Alva Hoosier
Earl R. Hoosier (1909-1973) Son of Alva & Pearl
Note: Cemeteries of Brown Co, IL, p. 534 has a listing of the descendants of Earl Hoosier and Beatrice Maude Hosford (b. 1912). If this is of interest to you, let me know.