Blood Relations

Concept:A family tree link: The phrase identifies the man's father as the only son of the woman's grandfather, revealing a sibling relationship.Explanation:Let the man be $M$ and the woman be $W$."Father of his brother" means the father of $M$'s brother, which is $M$'s father (call him $F$).So $F$ is the "only son of my (W's) grandfather".Thus $F$ is the only son of $W$'s grandfather.This implies $F$ is either $W$'s father or $W$'s uncle.• If $F$ is $W$'s father, then $W$ and $M$ are siblings (both children of $F$).• If $F$ is $W$'s uncle (brother of $W$'s mother), then $W$ is $M$'s cousin, but this option is not listed.Only the first case fits the given options.Thus $W$ is $M$'s sister.Answer:Sister