Why do Sister & Stella fight & carry on? Is it sibling rivalry?

Sister is at once paranoid and intensely jealous of her
sister, Stella-Rondo,whom she tells us is

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“exactly twelve months to the day younger than I
am and for that reason . . . spoiled.”

The meticulously skewed
logic apparent in this statement suggests the odd nature of the story that follows,
Sister’s tall tale explaining why she lives at the post office, or, as she notably
phrases it, “the P.O.,” for in Welty’s narrative voice—dialect, cliches,
non-sequiturs—and plot are virtually indistinguishable.

The
fivekey characters in Sister’s tale are her mother; Stella-Rondo and her
“adopted”daughter, Shirley T.; her grandfather, Papa-Daddy; and Uncle Rondo. These
encounters are set in motion by the jealousy reawakened in Sister by Stella-Rondo’s
return to her China Grove home with Shirley T. following her separation from her
husband, Mr. Whitaker, the taker of “‘Pose Yourself’ photos” from
Illinois.(Sister believes that Stella-Rondo stole Mr. Whitaker from
her.)

The encounters are related in
dialogues which contain contradictory accounts of assertions made by the unreliable
narrator, presenting multiple perspectives whose plots extend beyond the temporal frame
of July 4th and 5th, and spatially beyond Sister’shouse to the Mississippi community
represented by the China Grove post office and to Mr. Whitaker’s
Illinois.