Pointing out to a lady, a girl said, “She is the only daughter-in-law of the paternal grandmother of my…

The correct choice is: Mother

The girl's father's only son is her brother.

His paternal grandmother is the mother of the girl's father. Her only daughter-in-law is the wife of her son, the girl's father, so the lady is the girl's mother.

Because the grandmother has only one daughter-in-law, the lady cannot be an aunt (the wife of another son) or a sister-in-law, and the relation can be determined.

Hence the answer is Mother.