Your nephew's baby would be your grand-nephew or grand-niece. You would be their grand-uncle or grand-aunt.
they are cousins
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.
Your brother's children would be your nieces and nephews.
Your brother-in-law's children are your nephews and nieces.
Cristiano Ronaldo has four because she had a new baby
If it's your sister then your sister, if it's your brother's wife then you sister-in-law
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your borther-in-law is your grandchildren's great uncle. They are his great nieces and great nephews.