Defense of the aunt
I don't think Seita's aunt was a bad person. She helped them out when they needed it. You seem to forget she had a family of her own to take care of. And there wasn't food for everybody, her husband and her daughter tried to do their best in the situation while Seita was parasitizing them. Ok, if Seita had looked for a job, maybe he wouldn't have found it, and even if he had, money was worthless. But still, he could have tried, he could have shown his aunt he was trying, maybe not money nor food but at least helping her out with the chores, just showing that he cared.
Let's be honest, if you feared for your husband and your kid starving to death, would you really take care of some stranger's kids??? They were family, but distant, and even your own brother, son, daughter... counts as a stranger if you don't know him/her.
Remember, in the beginning she was nice. She wasn't always a b*tch. And you should try to understand her, she was really stressed by the war and the whole situation and the kid not helping started getting on her nerves and she started to almost hate him. At that point, when someone annoys you that much, you can't barely feel compassion. Frankly, I think that if they had come back to their aunt, and he promised to do something to help, she would have taken them in. Who would deny help to a poor dying child?? Not even her in her b*tchy days.
I know this because I have had to live with annoying family. In the beginning you are nice and you try to be patient and you talk to them very nicely and talk out the problem and fix the situation. But, after some time, if they don't change, if they don't care about you being uncomfortable in your own home in which you took them in, why should you care about them? I KNOW IT SOUNDS HORRIBLE, but let's admit it, we're not Jesus, and even the nicest of people eventually gets annoyed and says mean things we will regret (I'm sure after some time their aunt regretted it). I've seen it happen too many times.
I'm not saying she wasn't cruel. And I sure didn't like it (specially when she yells at Seita because Setsuko won't stop crying). But I can understand why she was that way. And I think most people in that situation would be the same or even worse.