What effect does the last sentence have on the impact of the story?
The last sentence of the story validates the premise of marriage being a private affair. The marriage between Nene and Nnaemeka is one that has worked out as a private affair. It did not necessitate the public intervention of the Ibo village elders and the larger social order, in general. While Okeke has rejected it for most of the narrative, the closing of the story compels him to accept the marriage as a private affair. The effect that the final sentence has on the story is to validate its premise and foster an epiphany that Okeke experiences as chilling.
Nene's letter is what compels Okeke to see his son's marriage in a different light. When she writes that her sons yearn to see their grandfather, it causes the old man to recognize that his public and demonstrative approach to marriage might not have been entirely accurate: "The old man at once felt the resolution he had built up over so many years falling in." When Okeke realizes that his own resistance has helped to cause the initial estrangement between he and his grandsons, it becomes too great of an emotional weight to bear. The emotional implications of such a separation begin to tread heavily on the old man:
Okeke was trying hard not to think of his two grandsons. But he knew he was now fighting a losing battle. He tried to hum a favorite hymn but the pattering of large raindrops on the roof broke up the tune. His mind immediately returned to the children. How could he shut his door against them? By a curious mental process he imagined them standing, sad and forsaken, under the harsh angry weather—shut out from his house.
It is in this where the last line becomes so powerful in its illumination of the poor decisions that the old man is scared that “he might die without making it up to them.” This sentence brings to light how much of a reversal Okeke has undergone. Its effect is to show how vast of a change has transpired within Okeke. From the father who cut out the wife of his new daughter in law in a picture from his son to the old man who "hardly slept" as a result of his actions, the effect of the last sentence shows this vast change within him in the understanding that marriage is a private affair.