One thing that might be useful in deciding upon a thesis statement for this story is to look at the major characters. Perhaps you could compare and contrast Sister with Stella Rondo. How are they alike? How are they different? You may find that there is a great deal of meat to this topic, and hopefully that will lead you to an argument concerning one of these characters. Perhaps, because we are never told explicitly, you could argue the parentage of Stella Rondo's "adopted child." Does the evidence from the story support Stella's claim that Shirley T. is adopted or does the evidence support Sister's claim that Shirley T. is Stella Rondo's real child? You could even address the bias of having one of the characters, Sister, tell the story....perhaps the story would be a bit different from someone else's point - of - view.
Hope those help!