One element of significance that Sanaubar's return demonstrates is the need "to become good again."  Sanaubar gave birth to Hassan and could not or did not choose to even hold him.  Yet, when she returns, she is able to serve as midwife to Hassan's wife, Farzana.  Her devotion to their son, Sohrab, is significant because it represents a desire to return and to make things right that had been miscalibrated.  It is also significant because it represents someone returning to do right by Hassan, something that Amir, himself, is going to replicate later on in the plot.  Another significance to her action is that it represents the idea of putting aside racial animosities to provide care and solidarity in a setting that necessitates it.  Even though she is Hazara and had been with a Pashtun, Sanaubar recognizes the need to be close to someone, anyone, as the same scarring that had been visited to her is now upon Afghanistan in a national setting.  The fact that she dies happy, in her sleep, knowing that she has found "a family," is also significant.  It is to this end that her return is significant, reflecting how there is a compusion to "be good" and this involves maintaining an eye towards the maintenance of social order.