There is no one listed in the 1798 census as Susanna Katharina, born in 1788. The only possible match is:
Family #116 Katharina Rudolf 1788
If Katharina Rudolf is not a match, then Susanna Katharina was probably born in another Volga village.
Susanna Katharina is not specifically mentioned in the 1857 census. Her husband had probably passed away, and she wasn't listed with her son's household, so there is the possibility that she remarried and lived in another family. There is also the possibility that she, too, passed away. Had she survived, he would have been 69 years old.
1834 Census of Grimm in the District of Saratov, Russia, dated 2 February 1835; Translated by Brent Mai, Concordia University, Portland, Oregon; Published by Dynasty Publishing, Beaverton, OR, USA; Published 2011; page 11, family #33, line 326, Susanna Katharina Hepp, age 46.
1857 Census of Grimm in the District of Saratov, Russia, dated 5 November 1857; Translated by Brent Mai, Concordia University, Portland, Oregon; Published by Dynasty Publishing, Beaverton, OR, USA; Published 2005; page 14, family #35.
The 1775 and 1798 Census of the German Colony on the Volga, Lesnoy Karamysh, also known as Grimm; Published by the American Historical Society of Germans from Russia, Lincoln, Nebraska, USA; Published date: 1995; family #119 in the 1798 census.