The answer to this question is a fairly simple one: Jeanne's mother blamed her father for remaining in the camp because Jeanne's father could not bypass his fear. Mama, herself, admits the following earlier in the book:
Mama's first concern now was to keep the family together, and once the war began, she felt safer there [Terminal Island] than isolated racially in Ocean Park.
Once this earmark had been achieved, however, Mama wasn't content at the camp. The family WAS together. Now the family WAS allowed to leave. She wanted desperately to go back home. However, Ko's fear is completely understandable! There were many hate-groups that had begun specifically against these Japanese Americans. These groups made it their mission to take farms away, commit hate crimes, drive families out of neighborhoods, and even ask for revocation of citizenship. Just hearing about these things was enough allow Ko to long for the safety of the camp.
Further, in regards to Mama and the idea of "blame," remember that all of these families were under a lot of stress both during wartime and after. Stress has a horrible way of bringing out the worst in people. Surely it did so here in the words and actions of Mama.
I would like to end my answer with a bit of irony. Before Ko and family are thrown into the internment camp, Ko had a very different idea about solidarity:
We were the only Japanese family in the neighborhood. Papa liked it that way. He didn't want to be labeled or grouped by anyone.
Originally, then, Ko enjoyed standing out so that he was not put into a "group" of Japanese Americans. I suppose he thought that, if he moved closer to a group of those like himself, he would be creating his own label. The irony is that, after the experience at the camp, Ko became more and more afraid of white Americans. Safety trumped it all.