The narrator in Rabindranath Tagore's short story Kabuliwala is a novelist trying to put together the chapters of his latest novel. He is constantly interrupted by his lively and much-loved young daughter Mini whose vivid imagination has her believing that the peddler whom she sees passing in the street may have little children in his knapsack. It is not long, however, before Mini and the peddler or kabuliwala become firm friends.
Being very inquisitive, Mini questions her father asking him how he and his wife are related. He decides not to explain the culture surrounding marriage to his little daughter, because the family is more progressive than his own generation and he also knows that, if he explains "the father-in-law's house" to little Mini, it will only invite many more questions which he has neither the time or inclination to answer. He quietly answers (but not for Mini to hear), "my dear little sister-in-law" and sends Mini off to play.
It is, no doubt, due to the same "differences of language in the world" that makes the doorkeeper call a "crow a krow..." that may create the confusion. The narrator's wife would have gone to her "father in law's house" when she married the narrator and, as he would be the son of her father in law, it makes her his sister in law, in its own context. There are other possible reasons why she could have actually been his sister in law who married him after the death of her husband, his brother, and so on, but such reasons are not given in this short story and it is therefore a cultural reference and a sign of respect that he calls his wife that.