The sons of your son-in-law's siblings are his nephews. You are not related to them.
However, your son-in-law is married to your daughter. The sons of her siblings are also your son-in-law's nephews. They are your grandsons.
Your nephew's son is your great nephew.
The child of your nephew is your great nephew (if a son) or your great niece (if a daughter).
Your nephew's daughter's son is your great-great nephew. Your nephew's daughter is your great niece.
Nephew
Your nephew.
A nephew is a cousin to his aunt's son.
Your nephew and grand nephew may be parent and child. In not, then they are First Cousins, Once Removed.
Your nephew's child is your great nephew (if a boy) or great niece (if a girl). You are his or her great aunt.
great nephew
brother
Cousin brother
niece/nephew