Without Obasan, there would be no story. Think about it. How do Stephen and Naomi even manage to grow up during such a tumultuous time? Who steps in when their parents both disappear and cannot take care of them? That's right: it's Obasan to the rescue.
Think of all the things that a mother should be (at least according to very traditional ideals): calm, caring, self-sacrificing, loving, and quiet. Until recently all women were expected to become mothers, so this list is also a bunch of ideal traits for women in general. Obasan ticks every box.
She's the perfect woman.
She is not wagamama, or selfish. Even though she is suffering too, all of her actions take care of other people. She sticks with Stephen and Naomi through thick and thin. Even Stephen's continuous rudeness can't send her away.
What's the big deal? Well the big deal is Kogawa centers her story on women, not men. Women are the strong ones in this here novel. Even Aunt Emily says:
The men are luckier than the women. It's true they are forced to work on the roads, but at least they're fed, and they have no children to look after. Of course, the fathers are worried but it's the women who are burdened with all the responsibility of keeping what's left of the family together. (14.114)
Women are the ones who have to keep the pieces of society together after the men are taken away. They have to raise the kids and hide their feelings. Just like Obasan.
So by titling this novel Obasan, Kogawa is not just talking about her own auntie. She's talking about all the women like her.