What is the theme of Obasan?

In 'Obasan,' the climax of the book comes near the end, where Nomi finally gets to read the packages of letters sent by Grandma from Japan. These letters tell of the horrific experiences during the Japan bombings and the aftermath of the explosion.

*Spoiler Alert*

Nomi reads about her family's fate in Japan and finding out why her mother was silent for so long. In the aftermath of the bombing, Grandma found Nomi's mother in a pile of rubble, disfigured and barely alive. Grandma herself wasn't severely injured, but both passed away because of radiation, extensive wounds, or lack of food and water.