I am currently trying to sort out the Forncrooks and have great sympathy for your delimma!
There is a Hannah Fornicrook(Forncrook) who married William Amock and produced John Aumock 7 Feb 1797.I believe she was Cristophel's sister as she was producing a family at the same time as Cristophel and Annetje. I found this record in "New York Births & Baptisms, Schoharie and Mohawk Valleys, 1694-1906.
I also found Sophia Forncrook m. Hendrick Van Horne, and Eva Forncrook m. Jacob Hoogeboom.They were also having children in the same period as Cristophel. Maybe more sisters.
I found more children attributed to Cristophel than I had previously had found on the internet.