Pointing to a woman in a picture, Amit said, Her granddaughter is the only daughter of my brother. How is the woman related to Amit?

The woman is Amit's mother.

The only daughter of Amit's brother is Amit's niece.

The woman is the grandmother of that niece.

The paternal grandmother of the niece is the mother of Amit and his brother.

So the woman is Amit's mother.