In the 1891 Census directory extract (from the 1890 census) it says there was a Viola S. (Straub), age 24 in his Son's, Joseph E. 's household. Probably his sister, cousin, or something?
DGM believes Erastus' mother & father are such based on telegraph operator (in 1870 Census, he's listed with Thomas & Matilde Straub), as well as 1860 Census. So, the 1880 Census must have had an error in his age.