Pointing to Ketan, Namrata said, “He is the son of my father’s only son.” How is Ketan’s mother related to Namrata ?

Namrata’s father’s only son – Namrata’s brother.

So, Ketan is the son of Namrata’s brother.

Thus, Ketan’s mother is the wife of Namrata’s brother i.e. Namrata’s sister-in-law.