Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
Second cousins.
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.
Great nephew
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Your niece's children are your great nieces and great nephews.
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
Your brother-in-law's children are your nephews and nieces.
Your brother's children would be your nieces and nephews.
Your nephew's children are your great nieces (girls) and great nephews (boys). You are their great uncle or great aunt, depending on your sex.
He is their Uncle and they are nieces and nephews.
Your brother's child's children are your grandnieces or grandnephews.
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.