Your son and your nephew are first cousins to each other. Your son's daughter and your nephew are first cousins once removed.
Your grandfather's nephew is your parent's first cousin. Your grandfather's nephew's daughter is your second cousin.
Your grand-nephew and your daughter are first cousins, once removed.
If your brother-in-law is the husband of your sister, presumably his daughter is your sister's daughter, too. Then his daughter and your nephew are either brother and sister or first cousins. If the daughter is not your sister's daughter, but the child of a previous marriage, then she is could be the half-sister of your nephew or not related to your nephew at all. If your brother-in-law is the brother of your spouse, then his daughter is your niece and your niece and your nephew are either siblings, first cousins, or not related at all, depending on how your nephew is related to you.
Your son and your nephew are first cousins to each other. Their daughters are second cousins to each other.
Your nephew's daughter's son is your great grand nephew (or great great nephew). You are his great great aunt if you are a woman, or his great great uncle if you are a man.
great nephew
The person is your nephew :)
Your daughter's daughter is your grand daughter. Your nephew's daughter is the granddaughter of your sibling (your brother or sister) The grandchildren of siblings are second cousins.
Your nephew's daughter is your great niece. Her son is your great great nephew. You are his great great aunt (if you are a woman) or his great great uncle (if you are a man).
Your sister's daughter is your niece, and her son is your grand nephew (or great nephew). You are his great uncle (if you are a man) or great aunt (if you are a woman).
You're not related, I've added a related link below explaing this.
Your nephew's daughter's son is your great-great nephew. Your nephew's daughter is your great niece.