Why does Stella-Rondo turn against Sister in the short story, Why I Live at the P.O.?

As the story opens, Stella- Rondo has separated from Mr. Whitaker and returned home with a two-year-old daughter she claims is adopted. Sister is convinced that the child is Stella-Rondo's and, by implication, that she got pregnant out of wedlock—a fact that the rest of the family has no interest in acknowledging.