Her aunt
The relation of the mother;s sister to daughter is niece-aunts.
Your mothers sister is your Aunt
None. She is your daughter's sister -in-law.
Your daughter-in-law's sister is not related to you, but is your son's sister-in-law.
Niece. Your mother has children, you and your sister. So your mother's daughter is your sister. When she has a daughter, this is your niece.
cousin
Being your sister's brother or sister you would be an uncle or an aunt to her daughter.
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.
Aunt Aunty Auntie
Your sister's son and your daughter are first cousins to each other.
Through marriage your step - sister's daughter is your step-niece!