Your nephew and your daughter are first cousins.
Aunt and nephew
They would be cousins.
Your nephew's daughter is your great niece and you are her great aunt (if you are a woman) or her great uncle (if you are a man).
Your daughter's son is your other daughter's nephew and she is his aunt.
You are his first cousin.
Great aunt or uncle.
Your son and your nephew are first cousins to each other. Their daughters are second cousins to each other.
Daughter in law
the boy is a grand nephew 2 me
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.
Well the daughter will be your grandaughter and you will be a grandmother/grandfathther.
In the United States there are no "legal relationship" between relatives, because no laws define these relationship. These are all customary terms. The child of one's niece or nephew is one's great niece (if a girl) or great nephew (if a boy). The elder relative is a great aunt or great uncle.