Family #242 in the 1834 Grimm census.
Family #194 in the 1857 Grimm census.
Maria Christina was born in 1808, after the 1798 census, and married before the 1834 census. This means she was not captured in a census with her biological parents. Unless we find a descendant who has detailed private records, we will probably never know her last name at birth.
Studying the maternal haplogroup of Maria Christina's female descendants could help narrow down her maternal ancestry.
1834 Grimm Census 
1857 Grimm Census 
Maria Christina was Johann Jakob Schneider's second wife in the 1857 Grimm census. There are only two possible people from the 1834 census whom she could be:
Tobias Eberhardt was married to a different, much younger woman at the time of the 1857 census. Usually this means that his first wife passed away, as divorce was uncommon in the Volga German community. This is not a likely match.
Jakob Alexander Schneider passed away in 1851 and his wife was no longer listed with his family in the 1857 census. 
This could mean she remarried or passed away. His wife in the 1834 census was named Maria Christina and her age is a match with the Maria Christina of this profile. These two references to Maria Christina appear to refer to the same woman.
As of the 1834 Grimm census, Alexander and Maria Christina had three children:
In the 1857 census which shows the death of Alexander Schneider and no wife, there are five children listed:
None of the birth years overlap with the children listed in the Johann Jakob Schneider household in 1857, a fact that helps support that she was the mother of all Alexander Schneider's 8 children, 3 from the 1834 census and 5 in the 1857 census.   Her three oldest children were old enough to be married and living in other households. With her youngest child born in 1848, she could not have married Johann Jakob Schneider until after that year.  Since we know her first husband Alexander Schneider died in 1851, she must have remarried to Johann Jakob Schneider after Alexander's death, perhaps as early as 1851 or 1852.
In the 1857 census listing for Maria Christina in the Johann Jakob Schneider household, the oldest child was 39 years old compared to her 50 years of age. Since it would be highly unusual to give birth at the age of 11, she cannot be that child's biological mother. Most likely she was not the mother of any of Johann Jakob Schneider's children. If she married Johann Jakob Schneider in 1851-1852, the oldest any child of theirs could be was 6 years of age. That does not match the age of any child in the family. It appears that she and Johann Jakob Schneider had no children together, at least none before 1857. Maria Christina was 49 years old that year, and the likelihood that the couple had children when she was aged 50 or older are slim.
One final note: Although her two husbands shared the surname Schneider, they were not from the same family line, therefore not related. It may be that some of their descendants are related through a later marriage, but as of 1857, there was no biological connection between these two family lines.
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