Your nephew's son would call you his great uncle or great aunt. Some would use the terms grand uncle or grand aunt.
your nephews
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
If it is a boy then it is your nephew. If it is a girl then it is your niece.
You would be his Great Uncle and he would be 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.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
Great nephew
Your brother's children would be called your nieces or nephews.
Your brother's children would be called your nieces or nephews.
Your brother's children would be your nieces and nephews. Your nephew's children would be great-nieces and great-nephews.
himselfhis brothershis first cousinsHis cousin in law once removed.
A cousin, in all those difficult cases they are your cousins