Sister is the narrator of this excellent short story who is shown through her narrative account of what happens to her and her family to be perhaps more prejudiced and biased than she herself is aware of. Sister is a woman who, at the start of the story at least, lives with her family and has a job as a postmistress in a Southern town. Her identity to a large extent is defined by her relationship with her younger sister. Her nickname is based on this relationship, and she is constantly comparing herself to her. Sister's presentation of Stella-Rondo in the first paragraph of this story is very interesting to analyse:
Stella-Rondo is exactly twelve months to the day younger than I am and for this reason she's spoiled.
Clearly, the fact that Sister shares her birthday with Stella-Rondo indicates the way in which she feels diminished because of this. The competition between them is something that dominates the entire story, but we can see that it was made worse when Stella-Rondo "stole" Mr. Whittaker from Sister and married him herself.
Throughout the short story we are left suspecting that Sister's presentation of the events is not completely true, and the way in which she presents herself as pitiful and more sinned against than sinning seems to rob her account of objective credibility. Note the way that she made "no bones about letting the family catch on to what I was up to" when she decides to move to the P. O. Her flagrant way of showing her unhappiness and her self-pitying speech and acts does not do much to endear her to the reader, and we are left thinking that she is a very spoilt and immature young lady.