“She is the only daughter of my father’s grandson’s father.” This is how Sia…

Concept: A chained relation statement (“X's Y's Z's ...”) is resolved by walking one relation-word at a time from the innermost person outward, replacing each link with the actual family member it points to, until the whole chain reduces to a single relationship with the speaker. Unless a sibling or other relative is explicitly named in the statement, each relation word is read along the speaker's own direct line — this is the standard convention for puzzles of this kind.

1. Start from ‘my father’: this is Ram’s own father.

2. ‘My father’s grandson’ is one generation below Ram’s father’s own children — following the speaker’s own direct line, that grandson is Ram’s own son.

3. ‘My father’s grandson’s father’ is therefore the father of Ram’s own son — which is Ram himself.

4. Substituting this back, the sentence “She is the only daughter of my father’s grandson’s father” reduces to “She is my only daughter.”

Cross-check: Reading the chain forward from this result confirms it — if Sia is Ram’s daughter, then Ram’s father’s grandson (Ram’s son) has Ram as his father, and Sia is that same person’s only sister, i.e. the only daughter described. Every link matches, so Sia is Ram’s daughter.