When Phoebe's father reads the note from his wife telling him she is going away, Sal has "a sinking, sinking feeling" because her own mother had gone away under similar circumstances, and had never come back.
Phoebe's mother is vague about why she is leaving, but it is clear that she is unhappy and needs to have some time alone to straighten things out in her own mind. Sal's mother had needed to do the same thing, and Sal has still not quite reconciled for herself exactly why her mother left and why she had not come back. Sal's feelings are in turmoil, and she is consumed by feelings of loneliness, betrayal, and even guilt. She refuses to talk to anyone about her unhappiness, however, preferring to avoid the issue because it is too difficult for her to face.
When Phoebe's father reads the note from his wife, it is kind of a situation of deja vu for Sal. The message brings back far too clearly the memory of how things were when her own mother left, a memory which she has been struggling mightily to forget. Sal knows how things will be for Phoebe's family now, how they will be confused and angry and hurt until the problem with Phoebe's mother is resolved. As close as she is to Phoebe, Sal also probably senses that now, she too will be forced to relive a time she would prefer not to remember (Chapter 20).