In decipher jumbled up description type of questions, a round-about description is given in the form of certain small relationships and direct relationship between the persons concerned is to be deciphered.
Introducing a man to her husband, a woman said, "his brother's father is the only son of my grandfather."
How is the woman related to his man ?
Only son of her grandfather -- Her father;
Man's brother's father -- man's father.
So, man's father is her father i.e., She is the man's sister.