Harish said on looking at the portrait of a man, "His mother is the wife of my father's son. I don’t have any brothers or sisters." Whose portrait was Harish was looking at?
His son
Since he has no siblings, Harish is the only child.
Wife of father's son -> His wife
His wife is mother of the person whose portrait he was looking at.
So the portrait is of his son (since the picture is of a man).