Mahoroba no Sora, Utakata no Kyoudai

Under an endlessly vast sky, in a tranquil townscape of tiled roofs, lived a close pair of siblings. Their parents ran "Inomoto Transport," and the older brother, Shohei Inomoto, a fourth-grader, was a kind and caring older brother. His younger sister, Mitsuko, a second-grader, had such a vivid imagination that she sometimes troubled her brother. However, they lived the ordinary lives of elementary school children. But then, a certain event leads them to wonder if "the world they live in" is "being controlled by someone's (God's) will." This faint thought eventually leads to…!?

(Source: Mangabu, translated)