Introducing a girl, Anthony said, "She is the daughter of the daughter of the mother of my aunt." How is the girl related to Anthony?

Let's break down Anthony's statement from the end:
1. "Mother of my aunt" $\to$ Anthony's maternal grandmother (or paternal grandmother).
2. "Daughter of the mother of my aunt" $\to$ This could be Anthony's aunt or Anthony's mother.
3. "Daughter of the daughter of the mother of my aunt" $\to$ Daughter of Anthony's mother (which is Anthony's sister) or daughter of Anthony's aunt (which is Anthony's cousin).

Among the given choices, "Sister" perfectly fits the relational structure.