Your nephew's daughter's son is your great grand nephew (or great great nephew). You are his great great aunt if you are a woman, or his great great uncle if you are a man.
Second cousins once removed.
Nieces are the daughters of your brothers and sisters. Nephews are the sons of your brothers and sisters.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Yes, there have been several popes that were related - one father and son and several popes had nephews who were elected as pope.
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.
They are cousins-in-law and are not related in any way by blood.
your nephews
They would be great nieces or great nephews.
Great nephew
3rd cousin
Fili and Kili are his nephews.
they are cousins