In Eudora Welty's "Why I Live at the P.O.," Welty does not write about her personal life. She had a positive relationship with her parents—they were particularly supportive of her work in writing.
Welty writes the story based upon what she sees traveling with her job. Visiting a post office in the South, she noticed a woman in the back room, ironing. We can imagine Welty being curious about a woman working at this facility (small, as we can imagine it would be), imaging that in doing domestic chores at "work," perhaps she lived there as well.
The relationship between our narrator "Sister" and her sister, Stella-Rondo could basically reflect the contrast between the siblings: with arguments and resentment. However, it is also suggested (in the eNotes Historical Context page) that Sister and Stella-Rondo are symbolic of different ways of life.
Sister has remained at home in her Mississippi home. Stella-Rondo married and left town, returning with their separation, with a two-year old child she insists is adopted. The contrast between the sisters is obvious. It is suggested, that Stella-Rondo represents a social trend in the North, across the nation where...
...a sense of a unified national identity and culture began to solidify.
This occurred in the North, but not in the South. We can imagine the struggle of the South to completely join in with what was happening in the North in terms of progress, as the South was still reeling from the effects of the Civil War, which changed life dramatically for this agrarian society that depended so much on slavery for its success. Once the South suffered widespread destruction in the war, and then needed to rebuild through the work of their own hands or paid labor—rather than slavery—society changed much more slowly, and not necessarily in the direction of the northern states.
More than any other region, the South retained a separate culture from the rest of the country.
However, is also noted that while the southern states could not remain independent from the nation, their defeat during the Civil War...
...served in some ways to strengthen regional identity.
Welty, a student of human nature and a sharp observer of the interactions between people, recognized this very different way of life...
One of the most important aspects of southern identity was the small town and rural lifestyle, with close-knit family and community at its center.
Enter the characters of the story. "Sister" represents the southern way of life--unable to understand how different life was for her sister. She is critical, and says to Stella-Rondo:
I think I would do well not to criticize so freely if I were you and came home with a two-year-old child I had never said a word about, and no explanation whatever about my separation.
Still unclear with regard to everything that happened to Stella-Rondo, Welty infers that life in the North...
...was fast-paced and dynamic but also alienating and isolating...
This drives a wedge between the two girls relative to Stella-Rondo's experiences—her:
...wider experiences in the North and her greater sophistication.
Their mother reminds them of how Stella-Rondo has changed:
But you must remember, Sister, that you were never married to Mr. Whitaker in the first place and didn't go up to Illinois to live...
Welty is pointing out the disparity in the sisters' lives, based upon changes to the face of the nation. These changes influence the attitudes of the sisters.