Pointing to a girl in a photograph, Kapil said, "My wife's son is the father of that person." How is that person related to Kapil?

Let's break down the statement made by Kapil step-by-step:
"My wife's son" refers to Kapil's own son.

"The father of that person" means Kapil's son is the father of the girl in the photograph.

Therefore, the girl in the photograph is the daughter of Kapil's son.

Thus, the person is Kapil's son's daughter.