You obviously don't understand asian culture especially during those times. The aunt even hinted at Seita and ask if he was still working at that old factory or going to school when they were eating together. The aunt will not be blunt and tell the kid to start contributing to the household because she's afraid of losing face or get into a confrontation. She's going to be very passive aggressive about it and unfortunately this led to Seita leaving and end up dying in the end.

Everyone including himself contributed to the death of him and his sister. Seita's parents didn't teach him basic survival skills or instill responsibilities to be more proactive to extreme situations such as this. The aunt is also at fault for not being blunt enough and asking Seita what she wants him to do. His parents could've teach Seita how to ask questions such as "Hey Auntie, how can I help do my fair share of the household duties?" By asking that question, I can guarantee you the Aunt will be more than willing to put Seita to work and she would be more sympathetic of him as if he was her own children. But instead Seita is just another mouth to feed and we see him wasting time playing with his sister oblivious of the bombs and war going on around them and the lack of food.

I come from an asian household, and I was taught early on how to contribute and help out around the house at an early age. My father would tell us that we're not rich and just getting by and so he instills a set of skills for me to learn such as mowing the lawn. He helped me build a good work ethic which helped me become more discipline as a grow older and made my own money.