This is a great question to think about, because so much of the richness of this story lies in the use of the unreliable narrator that Welty uses to great effect here. Initially, I think we are all struck by the injustice of what has been done to Sister. She spares no efforts to tell us how her sister manages to turn the entire family against her and force her to have to leave them so that she can live her own life and not be troubled any more by her sister's machinations against her. In addition, from what Sister says, there is obvious favouritism towards Stella-Rondo, who has been spoiled since birth:

She's always had anything in the world she wanted and then she'd throw it away. Papa-Daddy give her this gorgeous Add-a-Pearl necklace when she was eight years old and she threw it away playing baseball when she was nine, with only two pearls.

If we don't delve any further into what is really going on in this story, we therefore could safely conclude that Sister is unfairly treated by her family and Stella-Rondo. However, Welty means us to interrogate and examine this narrative very carefully. If we do so, on second reading, we begin to suspect that all is not as it may seem, and we begin to think that perhaps Sister is a character who is actually at least partially responsible for making her own difficult situation. She is somebody that is very difficult, and her obvious grievance against her sister leads her to antagonise not just her but also other members of the family as well. She seems quick to take offence when actually there is little offence meant. Therefore, there might be a distinct difference between your first impressions of Sister and her situation and the reality.