@Revvex It's possible it's the same world, both seem to have suffered a nuclear war. In
Jiyuu he no Toubou it sounds like her little sister suffered both radiation poisoning and burns, all of which happened the day the blue sky went away. (The burns are visible, and it mentions her body's tissue lost it's shape.) There being an explosion that day is mentioned as well. Also the little girl's wings are clearly some kind of mutation. This world sure looks like it's in the midst of a nuclear winter that started recently.
Since it's clear and sunny in the outside world of
Jiyuu he no Toubou, it's probably set many years after this story, as the nuclear winter appears to have ended.