Introducing a woman, Shashi said, "She is the mother of the only daughter of my son." How is that woman related to Shashi?

Let's break down the family relations step-by-step from the inner clause:
1. "My son" $⟹$ Shashi's son.
2. "The only daughter of my son" $⟹$ Shashi's son's daughter (which means Shashi's granddaughter).
3. "The mother of the only daughter of my son" $⟹$ The mother of that granddaughter, who must be the wife of Shashi's son.

The wife of Shashi's son is structurally related to Shashi as a Daughter-in-law.