If you look at her children in 1870, they are living with a man named M. H. Goddin. I was thinking perhaps a step-father. If you look in the same cemetery as Thomas Snow, there is a grave for a Mrs. Elvira Goddins, and another for an infant named Elvira Goddins. Perhaps, this could be your missing Elvira, and another possible child.
Edit to add there is a marriage between a woman named Elvira Snow and a Mortimer H. Godwin in 1859 in Polk, Texas.