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.
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.
They are your grand-nephews and grand-nieces or great-nephews and great-nieces.
Great nephew
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 nephew's children are your great nieces (girls) and great nephews (boys). You are their great uncle or great aunt, depending on your sex.
Your brother-in-law's children are your nephews and nieces.
Your brother's children would be your nieces and nephews.
He is their Uncle and they are nieces and nephews.
Your brother's children would be your nieces and nephews. Your nephew's children would be great-nieces and great-nephews.