You will be a great aunt or uncle.
Your nephew's baby would be your grand-nephew or grand-niece. You would be their grand-uncle or grand-aunt.
The child of your nephew is your great nephew (if a son) or your great niece (if a daughter).
Your sister's son is your nephew and you are their aunt or uncle. Their baby would be your great nephew. Some use the term grandnephew, but great is more accurate.
They are first cousins once removed.
if she has a boy, then its a nephew. its she has a girl its a niece.
Your nephew's daughter is your great niece. Her child is your great great niece or great great nephew.
Your sister's son is your nephew.
Your brothers baby boy is your nephew
Your Nephew :)
a niece or nephew
The grandson of your nephew is your great great nephew, but few people use that relationship as a form of address. Usually they call relatives by name or business title, not by relationship.
Your nephew's son is your grand nephew or great nephew. Some people say one, some say the other.