Family #48 in the 1834 Grimm census.
Family #33 in the 1857 Grimm census.
According to the 1834 census, Anna Regina's "mother" Christina had a son, Johann Friedrich Stehl, who was three years old in 1834. If Anna Regina was nine and a half years old, then Christina could not have been her mother. Her father must have been married at least once before he married Christina, and that woman was Anna Regina's mother.
Unfortunately Anna Regina's mother was born after the 1798 census and died before the 1834 census, so her name was never captured in an official record. At least not one yet identified. The 1816 census tallied males only.
Note that the inclusion of Christina Hepp's child by her first husband indicates that Christina was not the biological mother of the two oldest children, Anna Regina and her sister Margaretha, since they were older than Johann Friedrich Stehl. Youngest child, Heinrich Hepp, would have been Anna Regina's step brother, since neither child had a common parent.
Family # 48
Head of the Household Gotlieb Hepp, age 39 in 1816, deceased 1824
Wife Katharina Margaretha Hepp, age 62
Child #1 Gottlieb Hepp, age 26
Wife of Child #1 Christina Hepp, age 29
Grandchild #1 Anna Regina Hepp, age 9 years 6 months
Grandchild #2 Margaretha Hepp, age 7
Grandchild #3 Heinrich Hepp, age 1 year 6 months
Step-Grandchild #1 Johann Friedrich Stehl, age 3
Brother Johannes Hepp, age 30 in 1816, to household #33
Grandchild #10 Christina Barbara Aab, age 6 months
Child #3 Konrad Aab, age 28
Wife of Child #3 Louisa Barbara Aab, age 23
Grandchild #11 Johann Jakob Aab, age 3
Grandchild #12 Karl Jakob Aab age 1
Child #4 Heinrich Alexander Aab, age 24
Child #5 Christian Jakob Aab, age 18
↑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 15, family #48, Gottlieb Hepp family.
↑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 #33, Margaretha Aab family.