Family #130 in the 1798 Grimm census.
Family #177 in the 1834 Grimm census.
In the 1834 census, he is listed as the step son of Jakob Polst. This means his father died after 1798 and his mother remarried Mr. Polst. Mr. Post passed away in 1816. Maria Magdalena Zerber Rau Polst died at some point prior to 1834.
Wife of Child #1 Maria Magdalena Zerber Rau, age 23
Grandchild #1 Nikolaus Rau, age 3
Grandchild #2 Anna Katharina Rau, age 6 months
Child #2 Nikolaus Rau, age 27
Wife of Child #2 Maria Barbara Schmück Rau, age 24
Grandchild #3 Anna Elisabeth Rau, age 6 months
Child #3 Johann Georg Peter Rau, age 25
Wife of Child #3 Anna Barbara Schröder Rau, age 20
Child #4 Johann Georg Rau, age 20
Child #5 Konrad Rau, age 7
Child #6 Anna Elisabeth Rau, age 14
Child #7 Maria Magdalena Rau, age 12
Child #8 Anna Margaretha Rau, age 10
His father died before 1816 and his mother remarried to Jakob Polst. Unfortunately Mr. Polst died in 1816, shortly after the male-only census was taken that year. It's not clear how many years his mother outlived his step-father, but she was in fact deceased by 1834. Mr. Polst had no sons to take over his land, so step-son Nikolaus Rau became the head of the Polst household.
Head of the Household Jakob Polst, age 41 in 1816, deceased 1816
Child #1 Nikolaus Rau, age 38, step-son
Wife of Child #1 Katharina Rau, age 36, stepson's wife
Grandchild #1 Maria Katharina Rau, age 12 years 6 months
Grandchild #2 Katharina Margaretha Rau, age 9 years 6 months
Grandchild #3 Elisabeth Rau, age 9 years 6 months
Grandchild #4 Charlotta Rau, age 8
Grandchild #5 Charlotta Rau, age 5
Grandchild #6 Katharina Barbara Rau, age 6 weeks
↑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 #130 in the 1798 census.
↑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 53, family #177, Jakob Polst family.