Your aunt is your daughter's great aunt. Your daughter is your aunt's great niece.
The daughter of your aunt is your first cousin. The daughter of your great aunt is your first cousin, once removed.
Your aunt's daughter is your first cousin.
Her aunt
Your great aunt's daughter's children (her grandchildren) are your second cousins.
their aunt or uncle
you would be her aunt and she would be your niece
If she's not your mother, that would make her your Aunt.
Being your sister's brother or sister you would be an uncle or an aunt to her daughter.
Your nephew's daughter is your great niece and you are her great Aunt (or Great Uncle).Correction:Your nephew's daughter is your grand niece. You are her grand aunt or grand uncle.
She could be your grandmother. Or she could be your great aunt.
There is no relation between you, and your biological Uncle's wife's sibling's daughter. Your Uncle's wife is technically an "Aunt in law", though as routine courtesy is simply called "Aunt". Her brother/sister, if your family and her family are that close, could be regarded as an "Uncle/Aunt in law", as a stretch. Thus the daughter of him/her could be thought of as a "cousin in law" to you. But this is all a matter of courtesy, there is no biological or legal relation.
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.