Pointing to a boy in the photograph Reena said, "He is the only son of the…

Concept: In a blood-relation puzzle, when someone says “my grandparent’s only child,” that only child must be one of the speaker’s own parents — a grandchild descends from a grandparent through exactly one of the grandparent’s children, so if that child is unique, it is necessarily the speaker’s own parent. Any relation described one step further down (“the only son of that child”) therefore falls in the speaker’s own generation.

Application:

1. Reena’s grandfather has only one child, so that child must be Reena’s own parent — Reena has no other way to descend from this grandfather.

2. The boy in the photograph is described as the only son of that same parent.

3. Since Reena and the boy share the same parent, they are siblings; the boy being male makes him Reena’s brother.

4. The question asks for Reena’s relation to the boy, so Reena is his sister.

Cross-check: if the grandfather’s only child were anyone other than Reena’s parent, Reena could not descend from this grandfather at all, contradicting “my grandfather.” This confirms the only child must be Reena’s parent, and hence that the boy and Reena share a parent.

Result: Reena is related to the boy as his sister.