The confines of this "separate culture" create Sister's feelings of isolation that...
In Eudora Welty's "Why I Live at the P.O.," the rural setting of the story enables Stella-Rondo to hurt Sister.
More than any other region, the South retained a separate culture from the rest of the country.
The confines of this "separate culture" create Sister's feelings of isolation that come from Stella-Rondo's lies, especially because Sister lives away from a vast population or city. (She has no real friends.)
Sister is largely oblivious to the world outside of her family and community...some of [Sister's] tension with Stella-Rondo is based on Stella-Rondo’s wider experiences in the North and her greater sophistication.
Sister lives with her family in the backcountry of the South. Things are slow to change. She got her job at the post office because of her grandfather. Her family is really the only one that uses it. Her ability to work depends on her family sending or receiving mail—just as their acceptance provides her with a social outlet.
Stella-Rondo has been able to see life outside of the never-changing southern town where she and Sister were raised. Stella-Rondo has been liberated from the South—she married, moved to the North, and returned without a husband—along with a child she insists is adopted.
Sister talks too much.
"Of course I noticed at once she looks like Mr. Whitaker's side too. That frown. She looks like a cross between Mr. Whitaker and Papa-Daddy."
"Well, all I can say is she isn't."...
So the first thing Stella-Rondo did at the table was turn Papa-Daddy against me.
"Papa-Daddy," she says..."Papa-Daddy!" I was taken completely by surprise..."Papa-Daddy, Sister says she fails to understand why you don't cut off your beard."
Of course, none of what Stella-Rondo says is true, but Papa-Daddy is furious.
So Papa-Daddy says, "You'll never catch me setting foot in that post office, even if I should take a notion into my head to write a letter some place." He says, "I won't have you reachin' out of that little old window with a pair of shears and cuttin' off any beard of mine. I'm too smart for you!"
If Sister had kept her opinions to herself, Stella-Rondo would probably have ignored her. However, as Sister is not quiet, Stella-Rondo continues to exact her revenge. With a new sense of self that has come straight from city living, she makes sure everyone gets mad at Sister.
Rejected by her family because of her sister's lies about her, Sister moves to the post office.
...here I am, and here I'll stay. I want the world to know I'm happy.
In reality, moving out doesn't mean much. Sister's grandfather controls her job. And she is tied to her family for social interaction—otherwise, she is all alone...and this is dire punishment indeed, in any small town. Despite her isolation, with Sister's talk, it doesn't seem like her attitude will change anytime soon.