Arthur
The main boy's real name in "The Spiderwick Chronicles" is Jared Grace. He is one of the three Grace siblings, along with his twin brother Simon and their older sister Mallory. The story follows Jared as he discovers a hidden world of magical creatures after finding a mysterious book left by his great-great-uncle.
"Spiderwick" refers to "The Spiderwick Chronicles," a series of children's fantasy books written by Tony DiTerlizzi and Holly Black. The story follows the Grace children—Jared, Simon, and Mallory—as they discover a hidden world of magical creatures after finding a mysterious field guide in their great-uncle's estate, Spiderwick Estate. The narrative explores themes of family, adventure, and the consequences of interacting with the fantastical world. The series has also been adapted into a film and graphic novels.
Your grandmother's nephew would be your parent's first cousin. They would be your first cousin once removed. Since the child of your cousin is a generation away from you they are the once removed.
You are first cousin, once removed to the children of your great uncle.
Your sister's son is your nephew. Your uncle is your nephew's grand uncle or great uncle.
Your great uncle's granddaughter is your second cousin.
Your great uncle's grandchild is your second cousin. (Your great uncle's child is your first cousin, once removed and your parent's first cousin.)
Your great great great uncle is the brother of one of your great great grandparents. Your great great great uncle's son is the first cousin of you great grandparents, so he is your first cousin, three times removed.
The uncle's nephew would be first cousins with the uncle's children. So the nephew would be a first cousin twice removed with the great granddaughter.
If your uncle is someone else's great uncle, you and that other person are first cousins, once removed.
You and your uncle's great grandchild are first cousin, twice removed.
Your great uncle's children are your parent's first cousins and your first cousins once removed.