Pointing to man in photograph, a woman said, "His brother's father is the only son of my grandfather". How is the woman related to the man in the photograph?

The woman says: "His brother's father is the only son of my grandfather."

We simplify this step-by-step.  “My grandfather’s only son” → That would be the woman’s father, or her uncl,e since her grandfather has only one son, and the woman's mother might be the grandfather's daughter.  So the sentence becomes: “His brother’s father is my father or uncle”  Now, let’s look at “his brother’s father”: The man's brother's father is just the man's father. So, the man’s father is the woman’s father, or the woman's mother and the man's father are siblings. This means that either the man and the woman share the same father, so they are siblings, or their parents share the same father which would make them first-cousins. 

From the options given, the woman is the man's sister is the only appropriate choice here.