Your great nephew's child is either your great-great niece or your great-great nephew, depending.
The children born to your siblings are your nieces and nephews.
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
They are your great nephews and/or great nieces. Alternately, some people say grand-nephews, grand-nieces.
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 nephew's child is your great niece (if a girl) or your great nephew (if a boy).
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Your nephew's children are your great nephews and great nieces (or grand nephews and grand nieces).
You can refer to your sister's children as your nieces and nephews. If your sister has daughters, they are your nieces, and if she has sons, they are your nephews. Collectively, you can call them your nieces and nephews or simply refer to them as your sister's kids.
your nephews
Nephews