Your nephew's daughter is your great niece and you are her great aunt (if you are a woman) or her great uncle (if you are a man).
Your daughter is you great nephews great cousin if u live in Australia ,USA ,UK
Your daughter and your nephew are first cousins. His daughter is your daughter's first cousin, once removed.
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
Great nephew
He is their Uncle and they are nieces and nephews.
Your nephews' children would be your children's cousins. They share a common grandparent, as their parents (your nephews) are siblings to your children’s parents. This familial relationship creates a bond among them, typically fostering a close connection within the extended family.
Second cousins.
Second cousins once removed.
It does not appear that Thomas Jefferson had any nieces or nephews. He had a daughter, a son in law and two grandsons. He also had an aunt and a first cousin.
Your brother's children would be your nieces and nephews.
Your brother-in-law's children are your nephews and nieces.
A cousin, in all those difficult cases they are your cousins