Your niece and your son are second cousins. Your niece's child and your son are 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.
The children born to your siblings are your nieces and nephews.
Your brother's grandson is your great nephew.
Julius Caesar's dad's name was also Gaius Julius Caesar. The Romans named their eldest son after the father.
You call them your nieces, or by their names.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Second Cousin
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Your niece's son is your grand nephew, and you are his great aunt.
Nieces.
If it is a boy then it is your nephew. If it is a girl then it is your niece.
Your niece and your son are second cousins. Your niece's child and your son are second cousins once removed.
Your brother's daughters are your nieces
Your brother's daughters are your nieces
u call your dads brother uncle in English