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 son-in-law is the husband of your daughter. His son is your daughter's son and therefore your grandson, and you are his grandparent.
If your son-in-law's son is from a previous marriage, the son is not physically related to you. However, you may want to treat him like a grandson anyway, especially if he came into your life at an early age. He is your daughter's step-son, he is your step-grandson.
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.
Your nephew.
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.
brother
cousins.
niece/nephew
Cousin brother
nephew