Pointing to the photograph of a girl, Sukanya said, "She is the daughter of the only daughter of my mother." How is Sukanya related to the girl in the photograph?

Sukanya is the girl's mother.

"The only daughter of my mother" is Sukanya herself.

So the girl is the daughter of Sukanya.

The relation of Sukanya to the girl is that of a parent.

Sukanya is the mother of the girl.