Why does Sal seem to notice that there is something wrong with Mrs. Winterbottom, but Phoebe doesn't?

Since her mother left, Sal has become more sensitive to others, rather than just concerned about herself. She reflects back on the events that preceded her mother’s leaving. This makes her aware of some similar unhappiness and dissatisfaction in Mrs. Winterbottom. She sees how Phoebe, her sister, and her father almost ignore Mrs.Winterbottom, who is desperately trying to reach out for someone to notice her. She talks a great deal at the dinner table, even though no one answers her. Sal notices this, and perhaps she sees her mother’s reaching out before she left. Sal wonders if she was exactly like Phoebe, and if this was a reason that her mother left, through not feeling fulfilled within her family. Mrs. Hiddle has suffered loss, as revealed later in the story. In hindsight, Sal realizes that she should have seen how unhappy her mother was. She wants to warn Phoebe, but does not know how to break through Phoebe’s self-centeredness to do so.