I have little doubt that Guisbert is closely related to the "Three Brothers, Stephen, William, Geysbert Van Sickle" mentioned in John Waddell VS's book from 1880 [page 183]:
"Geysbert Van Sickle, brother of Stephen and William, had seven children; I. James; II. Jonathan; III. Chisters; IV. Geysbert, whose children live in Woodbridge and Metuchen, N.J.; V. Mary Ann; VI. Phoebe; and VII. Lydia Ann, who married Otis D. Stewart, of New Brunswick, N.J."
The book indicates that this Geysbert, brother of Stephen and William, is the son of Reinier VS and Stijntje (online sources cite that she was born Stijntje SUYDAM). Reinier's will seems to confirm this:
"1749, August 31. Van Sickelen, Reinier, of Reading Township, Hunterdon Co.; will of. Wife, Styntie. Daughter, Jannetie. Residue to wife and sons Ferdenant and Ryke, Geisbert and Hendricks, and daughters Jannetie and Marya. Exercutors- brothers-in-law, John Sudam, and brothers Geisbert and Johannis van Sickel. Witnesses- John Van Sickle, Peter Van Sickle, John Van Boskerck. Proved March 6, 1749-1750."- Calendar of New Jersey Wills, Vol. 30, Page 502.
Note that the will mentions that Reinier had a brother names Geisbert. It is merely a guess, but I believe your Guisbert is either Reinier's brother, or a descendant of Reinier's brother.
I must admit that this line is very confusing, and I have been researching them for quite some time. I believe my family line ties into this branch in some way (my ancestor may be the William mentioned in the will that you provided).
I hope this has been helpful. Thank you so much for posting the will!
P.S. I do have info. on the Woodbridge Van Sickles mentioned in John Waddell VS's book, as well as some other tidbits on the Guisbert/Gysbert name that I've gathered over the years. Please contact me directly if you're interested.