The answer is: herself or one of her sisters. Her mother's son is her brother, and she and any female siblings would be her brother's son's aunt.
If the son married his aunt
Here is an example: Say a family has a mother, father, two sons and a daughter. The one son is married with a child. Both sons are in the army fighting a war somewhere and both get killed. The mother and father have lost 2 sons, the daughter 2 brother, the one sons wife a husband and a brother-in-law, and her baby a father and an uncle. Then the wife of the one deceased son might marry someone else and move far away. Now the mother and father have lost a grandson and daughter-in-law, the daughter an aunt and cousin. It is also not uncommon for a wife to leave her husband (for various reasons) while he is at war somewhere.
Your aunt's child, son or daughter, is your First Cousin. The same is true for your uncle's children.
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
Your mother's sister is your aunt. Your aunt's daughter is your first cousin. Any child of your first cousin is your first cousin, once removed.
If the son married his aunt
Your mother's aunt's husband's sister is your mother's aunts sister-in-law but is not related to you. Her son is not related to you either.
Aunt.
Your mother's aunt's is your mums first cousin and your first cousin, once removed.
Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
Mother and Son - 1984 The Aunt 2-5 is rated/received certificates of: Australia:G
Your aunt's daughter is your first cousin, and her son is your first cousin, once removed.
Great Aunt.
Your aunt's only sister is your mother, and you are her son or daughter.
step brother
Your niece's son's son is her grandson. He is your great great nephew and you are his great great aunt or great great uncle, depending on your gender.
Your mom's sister is your aunt, and her son is your first cousin.