Georg Heinrich Wittmann's father was no longer living in 1857. To determine which Wittmann brother was his father, I compared the sons of his grandfather and determined which one was no longer living at the time of the 1857 census. Only Michael is unaccounted for, meaning he was the father of Georg Heinrich and his three brothers.
Child #1 Christian Jakob Wittmann, age 24 years 6 months
Child #2 Katharina Elisabeth Wittmann, age 18
Child #3 Elisabeth Wittmann, age 16
Brother #1 Konrad Wittmann, age 42 in 1850, to Rosenberg in 1852
Child #4 Valentin Wittmann, age 21 in 1850, to Rosenberg in 1852
Child #5 Michael Wittmann, age 19 in 1850, to Rosenberg in 1852
Child #6 Johann Georg Wittmann, age 17 in 1850, to Rosenberg in 1852
Child #7 Johann Friedrich Wittmann, age 3 in 1850, to Rosenberg in 1852
Brother #2 Unnamed Wittmann, probably Johann Michael Wittmann, probably deceased before 1850 and his death noted in that male-only census
Child #8 Georg Heinrich Wittmann, age 18, nephew by unnamed brother
Child #9 Johannes Wittmann, age 16, nephew by unnamed brother
Child #10 Heinrich Wittmann, age 14, nephew by unnamed brother
Child #11 Konrad Wittmann, age 12, nephew by unnamed brother
↑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 30, family #75, Johann Wittmann family.