Your sister in law's son would be your nephew. If he was not your brother's biological son, I guess technically he would be your step-nephew. A niece would be if the child were a girl.
Your nephew's son is your great nephew. Your niece's son is also your great nephew.
Your nephew's son is your grand nephew or great nephew.
Your nephew's son is your great nephew.
Your nephew's son is your great nephew.
Your wife's nephew is your nephew and his son is your wife's, and your, great nephew.
If your sister had a son, then he is your nephew.
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
You're a Great Aunt to your nephew's son, who is your great nephew.
Your nephew's son is your grand nephew or, some would say, your great nephew. You are his great aunt or great uncle.
The relationship sequence:Your nephew/niece is a child of your sibling and your son's first cousin.Your nephew/niece's son is your great nephew (or grand nephew) and your son's first cousin once removed.The grandchild of your nephew/niece (your great nephew's son) is your great great nephew and your son's first cousin twice removed.
Your brother's (or your sister's) son is your nephew and you are his aunt if you are a female or his uncle if you are a male.