It is impossible to tell whether Momma carrying Sal is what caused the baby to die—the stress of lifting Sal could have caused complications, or the baby’s death may have been entirely unrelated. Either way, though, the stillbirth isn’t Sal’s fault—she didn’t break her arm on purpose, nor could she have known that Momma would carry her. But the reader has already seen that Sal she deals with grief by blaming herself, so she assumes that she must have caused her sister’s death. This mirrors Phoebe’s practice of telling wild stories—there’s a remote chance that Phoebe is right that Mrs. Winterbottom was kidnapped, but it’s highly unlikely and impossible to verify. Nevertheless, it makes Phoebe feel better and more in control, just as Sal tries to understand the stillbirth by blaming herself.