With the aide of a lawyer and the courts.
A niece or nephew is the child of an aunt's or uncle's brother or sister. So the parents of the aunt/uncle and of the niece's/nephew's parents are the same people - their grandparents. The grandmother of the aunt or uncle is the great grandparent of the niece or nephew.
The godparents might.
Your aunt's husband is your uncle. You are his nephew if you are a boy, niece if you are a girl.
You're a Great Aunt to your nephew's son, who is your great nephew.
Your uncle is the brother of one of your parents or the husband of the sister of one of your parents. Your aunt is the sister of one of your parents or the wife of the brother of one of your parents. You are their niece (if you are a girl) or nephew (if you are a boy).
The Aunt would need to file a motion for custody of a child in need of care.
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
a nephew's kid to his aunt's kids would be their cousin.
You are your nephew's uncle, or aunt, and his son's great uncle, or great aunt. Your nephew's son is your great nephew.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Your nephew's child is your great nephew (if a boy) or great niece (if a girl). You are his or her great aunt.
You are a great great Aunt (or Uncle) to the grandson of your Nephew.