Jayant who is the only son of his father pointed to the photograph of a kid…

Concept: In a chained blood-relation statement such as ‘my X’s Y’s Z’, unpack one relation at a time, starting nearest to the speaker and working outward, before naming the final relative. Two identities matter here: a person’s ‘son’s sister’ is that person’s own daughter, since a son and his sister usually share the same parents; and a married person’s ‘father-in-law’ is their spouse’s father, not a blood relative of their own.

1. ‘My son’s sister’ is Jayant’s daughter, because Jayant’s son and his sister share the same parents.

2. That daughter’s mother is Jayant’s own wife.

3. Jayant’s wife’s father-in-law is her husband’s father, that is, Jayant’s own father.

4. So the phrase identifies the photographed kid as a grandson of Jayant’s father.

5. Jayant is stated to be the only son of his father — this detail only does useful work if the ‘grandson’ being pointed to is read as one produced through a son (a grandson via a daughter would make the ‘only son’ fact irrelevant to solving the puzzle), so the intended reading is the grandson through Jayant’s own line.

6. Since Jayant is his father’s only son, that grandson-through-a-son is Jayant’s own child.

7. A kid described this way is therefore Jayant’s own son.

Cross-check: If the boy is Jayant’s son, then Jayant’s father is the boy’s grandfather, and Jayant’s father’s grandson through his only son, Jayant, is exactly the boy in the photograph — matching the original statement, and matching the Hindi original’s precise word for this relation, which names a grandson specifically through a son.

Answer: Jayant is the boy’s father.