What symbolism is there in "Why I Live at the P.O."?

You have to look at the story in its entirety to see the Biblical allusion to the story of the Prodigal Son. The narrator does not especially like her sister because she has just returned after leaving under suspicious circumstances. The sister is furious that she has to cook two chickens and try to feed five people plus one small child just because her "spoiled sister" has come home. Like the brothers in the Biblical story, the narrator is probably jealous because her sister left under disgraceful circumstances and is now being welcomed back to the family, with a meal which reminds us of the feast in the story of The Prodigal Son. The spoiled sister's "sin" seem to be quickly forgiven, just as the wandering son's sins were forgiven by the father in Biblical story.