Sal is both drawn to and repelled by Phoebe. She understands Phoebe's incredible grief at her mother's disappearance, but she is also so frustrated by Phoebe's stiff and ornery ways. Phoebe doesn't treat her mom with respect or show her love, which at times upsets Sal because she, too, has recently lost her own mother. Like Phoebe, Sal treated her mom poorly right before her mom left, and she regrets this behavior very much.
Sal can see things Phoebe can't, which highlights Phoebe's obliviousness. But it's also true that her friendship with Phoebe helps illuminate things in Sal's life, too:
Phoebe and her family helped me, I think. They helped me to think about and understand my own mother. (44.12)