Ravi said to Aman, "The girl who is reading a book is the only sister of my two sons." How is the girl related to Ravi?

Let's break down the statement made by Ravi:
* "My two sons" refers to Ravi's sons.
* "The only sister of my two sons" means the sister shared by Ravi's sons.

The sister of a person's sons must be that person's own daughter. Therefore, the girl is Ravi's daughter.