According to Sister, how did Stella-Rondo steal her boyfriend in the short story, Why I Live at the P.O.?

Sister had dated Mr. Whitaker first, but Stella-Rondo broke them up and married him herself. Sister says that Stella-Rondo won him unfairly by telling the lie that she is bigger on one side than the other. This interpretation seems inadequate and the existence of Shirley T. suggests that his decision might have had more to do with the fact that Stella-Rondo had premarital sex with him and became pregnant.