Daniel,
I did a little more digging and found Gottlieb’s obituary. You should be able to view it from this link.
https://www.newspapers.com/clip/34011029/gottlieb_zerbst_obituary/I also found the records for his two sisters births. They are as follows:
Name: Wilhelmine Albertine Zerbst
Gender: Female
Birth Date: 3 Jul 1858
Baptism Date: 11 Jul 1858
Baptism Place: Schwersens,Posen,Prussia
Death Date: 14 Jul 1858
Father: Gottlieb Zerbst
Mother: Juliane Kowalska
FHL Film Number: 764053, 764054
Name: Henriette Auguste Zerbst
Gender: Female
Birth Date: 6 Dez 1862 (6 Dec 1862)
Baptism Date: 1 Jan 1863
Baptism Place: Schwersens,Posen,Prussia
Death Date: 17 Mai 1864 (17 May 1864)
Father: Gottlieb Zerbst
Mother: Juliane Kowalski
FHL Film Number: 764053, 764054