While looking at a photograph, Arvind pointed to a woman and said, "She is the mother of my wife's only son." How is the woman related to Arvind?

Let's break down the statement made by Arvind:
1. "My wife's only son" refers to Arvind's own son.
2. "The mother of my wife's only son" means the mother of Arvind's son.
3. The mother of a man's son is that man's wife.

Therefore, the woman in the photograph is Arvind's Wife.