Aunt or uncle would be the most common term. The actual term would be great aunt or great uncle.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
He would be your grand nephew; you would be his great-aunt.
Your sister's son is your nephew and you are their aunt or uncle. Their baby would be your great nephew. Some use the term grandnephew, but great is more accurate.
The son of your brother or sister is your nephew.The grandson of your brother or sister is your great nephew. (Some would call him your grand nephew.)The great grandson of your brother or sister is your great great nephew. (Some would say your great grand nephew.)
Your great great nephew's child is your great great great niece or great great great nephew, depending on the sex of the child.
Your great nephew's child is either your great-great niece or your great-great nephew, depending.
You would be his great aunt. To you he would be your great nephew.
Some would call your nephew's children your grand nephews and grand nieces, others would call then your great nephews and great nieces.
Your nephew's son would call you his great uncle or great aunt. Some would use the terms grand uncle or grand aunt.
Your brother's child's children are your grandnieces or grandnephews.
you would be the child's great aunty or uncle Therefore, they would be your great niece or nephew.
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.