There's a very slight risk of inbreeding, but it's really not as bad as most people think.
In many cultures, first cousins are regarded as ideal mates.
brother
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 mothers niece is your first cousin. Your first cousin's child is your first cousin, once removed.
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.
Same way as human mothers
Your brothers baby boy is your nephew
Your Nephew :)