Pointing to a boy in the photograph Reena said, "He is the only son of the only child of my grandfather." How Reena is related to that boy?

Explanation:-

The boy in the photograph is the only son of Reena's grandfather's only son; i.e., the boy is the only son of Reena's father.
Hence, the boy is the brother of Reena or Reena is the sister of the boy.