Pointing to a lady , a man said," The son of her only brother is the brother of my wife."How is the lady related to the man ?

Brother of person's wife → brother-in-law of the person. Hence, the son of lady's brother is brother-in-law of the person.

Therefore, the brother of the lady is the father-in-law of the person. Hence, the lady is the sister of the person's father-in-law.