To get to theme, home in on the details of narrative voice and setting to see what larger ideas rise to the surface.
NARRATIVE VOICE: Sister tells the tale, and the first question one must ask of a first-person narrator is, What kind of a person are we dealing with and the second question is, Is she a reliable narrator?
Who is she? The first paragraph will tell you everything in what she says about others. How does she characterize her sister, Stella-Rondo and their relationship? Full of conflict and lies. When a character begins a story telling you how a fight is all someone else's fault, don't red flags go up, making you wonder if the person is telling you the whole truth? Take the phrase, "deliberate, calculated falsehood": Sister doesn't just say, "My sister lies"; she says, "She deliberately and calculatingly lies." It's a voice that makes its point in pedantic, teacherly style, driving the point home, and demonstrative of someone who thinks she's always right. The paragraph ends with a judgment -- that Stella-Rondo is "spoiled." When we're getting a confrontational, ugly tone already from Sister, accusations flying, and we've only just met her, don't we get a bit suspicious?
I advise you to home in on several paragraphs this way to find evidence of a judgmental, accusatory voice, Sister who is always wronged, always vengeful, and always in the middle of some dust-up with a relative. Note she has tiffs if not all-out wars with at least 4 people in the story.
So, already we are on the cusp of theme: judgmental attitude, sibling rivalry, reliability, lies, pettiness, small-mindness, myopic views....note I've mentioned several big ideas. There isn't just one, but you may be able to link a few to get to a larger statement of theme. See the eNotes link below for more discussion.
SETTING: Back to the ideas of reliability and lies: It's not to say that Stella-Rondo isn't spoiled or the favorite child or that Sister is a complete liar. In fact, the overall setting confirms some of Sister's accusations. Remember that setting has many components: geographical location, time of day, time of year, century and historical period, and physical locations inhabited by characters (natural or manmade). There's also emotional atmosphere that is generated by these subcategories of setting. The setting is artfully constructed by Welty to show us a small-minded place where tempers flare as a matter of course. Let's look at a few aspects where you can draw out evidence of ideas in action.
Physical Location: China Grove, MS is a small town. (Do small-minded people reside there?) What details tell you that the place is set in its ways, not too diverse, and puts people in boxes?
Time of Year: It's summer and "hotter'n Hades" (not a quote, but someone might well have said it). Fourth of July, too, where people celebrate by setting off...what? Do you see how heat and explosives contribute to an emotional atmosphere of ....? Fill in that blank with a descriptive phrase.
Century/Historical Period: How do we know it's not the South of 1900, 1960, or 2009? (It's 1941. Where was technology, where were race relations, what world events were occurring?) There's radio, there's Jim Crow segregation, and WW I is happening. Think about how aspects of daily life such as these three elements contribute to theme.
We are just scratching the surface. Dig deep into individual lines and scenes and you'll find theme.