pointing to gopi, nalni says, i am the daughter of the only son of his grandfather. how nalni is related to gopi?

Nalni says, "I am the daughter of the only son of his (Gopi's) grandfather." Breaking this down:

• Gopi's grandfather's only son is Gopi's father.
• Nalni is the daughter of Gopi's father.
• Therefore, Nalni is Gopi's sister.

Hence, Nalni is related to Gopi as his sister