The lady states: "Your father's wife is the only sister of my brother"
Breaking down the relationship:
"Your father's wife" refers to the man's mother.
"The only sister of my brother" refers to the lady herself (she is the only sister of her brother).
The statement establishes that:
The man's mother = The lady
If the lady is the man's mother, then the man is the lady's son.
Therefore, the answer is Option 4: Son.