There was a sister, Therese Jacobi, and a third brother, Eduard Jacobi, who was younger than Carl. Eduard did not pursue an academic career, but followed instead his father's profession as a banker.
Nagkaroon ng isang kapatid na babae, Therese Jacobi, at ang isang ikatlong kapatid na lalaki, Eduard Jacobi, na mas bata sa Carl. Eduard hindi ituloy ng isang academic karera, ngunit sinundan sa halip kanyang ama propesyon bilang isang bangkero.