Pointing to a woman, Rahul said, 'Her son's father is the only son-in-law of my mother.' How is Rahul related to the woman?

The woman's son's father is her husband. He is the only son-in-law of Rahul's mother, so he married Rahul's mother's daughter — i.e. Rahul's sister. That daughter is the woman herself, so the woman is Rahul's sister and Rahul is her brother.