Mother's daughter's daughter, perhaps

NIECE

Your sister’s child might call you this, but it’s not your son or daughter.

This clue plays with family relationships in a slightly indirect way. "Mother's daughter" is the mother herself (since she’s her own mother’s daughter), and her daughter would be the mother’s granddaughter—or, from another perspective, her niece if the daughter belongs to her sibling. The word "perhaps" hints that this isn’t the only possible answer, but "niece" fits neatly as the most common interpretation.