sweety
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
If it is a boy then it is your nephew. If it is a girl then it is your niece.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
Your brother's children would be your nieces and nephews. Your nephew's children would be great-nieces and great-nephews.
Your borther-in-law's children are your nieces and nephews. You are their uncle or aunt, as is your spouse.
He would be your grand nephew; you would be his great-aunt.
The English language has no collective term that would cover both nieces and nephews, other than the general word "relatives." According to Merriam Webster & Wikipedia; 'niblings' (play on siblings) is a collective term for nephews and nieces.
Your nephew's children are your great nephews and great nieces (or grand nephews and grand nieces).
where is the real niece and nephew of Bernie Mack
You call them your nieces, or by their names.
There is no special term in the English language to describe a relationship between you and your nephew's wife. To her, you are just "my husband's uncle." If she has children with your nephew, they will be your great nephews and great nieces and you will be their great uncle or great aunt.
Staying with Aunt Wanda