Thaisha's decision to leave her family to help rebuild the world reminds me so much of the story of Satre's student that he uses to explain existentialist philosophy in existentialism is a humanism. The story goes that a student of Satre's came to him for advice during WWII to ask if he should go fight in the war (to fight for his country and justice, avenge his brother's death) or stay with his mother (who was grieving and had no one left but him). Sartre tells him "You are free, therefore choose, that is to say, invent. No rule of general morality can show you what you ought to do." Whichever choice the student makes, will indicate which he felt was more important. To greatly oversimplify existentialist philosophy, the meaning of life is made through the choices we make. There are no correct choices, only choices made with full responsibility for the choice itself.
Thaisha was in an analogous situation. There was no correct choice between rebuilding the world or raising her family. There was only the choice made with responsibility, the choice she could live with. By embracing the fact that she did make a choice to leave, and accepting the consequences as such, she is acting in good faith, and living a meaningful life.
