In "Why I Live at the P. O." how can we know that the child is Stella's?
We can't. We need to remember that one of the most important aspects of this narrative is the way that it is told from the point of view of an unreliable narrator. As such, it becomes incredibly difficult to work out what is true and what is not from Sister's account. Stella-Rondo claims that her daughter, Shirley-T., is adopted, and this is something that appears to be accepted by the rest of the family. It is only Sister who has her suspicions that she is not adopted and that Stella must have conceived Shirley-T. before she was married. The only proof that Sister has to suggest this is the semblance that Shirley T. has to Papa-Daddy:
I said that oh, I didn't mean a thing, only that whoever Shirley-T. was, she was the spit-image of Papa-Daddy if he'd cut off his beard, which of course he'd never do in the world.
Apart from this, no further proof is offered either way. The only thing we need to remember is that Sister, as an unreliable narrator, seems very bitter about the way that Stella's return has replaced her in her family's affections and she seems to be willing to do anything to discredit her.