fourth cousin
The child of your nephew is your great nephew (if a son) or your great niece (if a daughter).
Your granpa's brother's son's son is your 2nd cousin.
Since your great grandfather was brother to the other person's great grandfather, the two great grandfathers had at least one parent in common. That parent is your common ancestor, the great great grandparent of both of you. People who share a grandparent are first cousins. People who share a great grandparent are second cousins. People who share a great great grandparent are third cousins. You are third cousins.
Frederick Prince of Wales, son of George II, father of George III ;-)
Your son and your great nephew (brother's daughter's son) are first cousins, once removed (i.e there is one generation separating them in the family tree) .
What is your great-great-great grand nephew?!
cousin
3rd cousin
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
your cousin
Your son would be the uncle of his brother's son.
My great great second, half, brother in law who's great great grandfathers....son.
Your grandmother is first cousins with cousin. Your parent would be the second cousin of the cousin's son. That would make you second cousins once removed.
Your nephew's son is your great nephew.
Great uncle / Great nephew
Great aunty
great nephew