Pointing to Manju, Raju said, "The son of her only brother is the brother of my wife". How is Manju related to Raju?

We can always break down such questions.

"The brother of my wife": This is Raju's brother-in-law.

"The son of her only brother": This refers to the son of Manju's brother, which is Manju's nephew.

Hence, Raju's brother-in-law is Manju's nephew.

Raju's wife and his brother-in-law share the same father. Since Raju's brother-in-law is Manju's nephew, his father must be Manju's brother. This means the father of Raju's wife is Manju's brother. The father of your wife is your father-in-law.

So, Manju's brother is Raju's father-in-law. This makes Manju the sister of Raju's father-in-law.