Dungeon Craft in the closet

I, Toru Shiina, a high school boy, found that a dungeon portal has been created in a my dorm room closet. Inside, the dungeon is as vast as a video game, and when I reached the bottom, under a large tree in the heart of the dungeon, my eyes widened in surprise. Because my younger sister, A-chan, who had died in an accident when we were children and whose body was never found, was sleeping there under the tree. From that day on, A-chan and I became masters of the dungeon in the closet, and we watched over the dungeon's growth. Today, I will go into the dungeon once again, to see my sister who is supposed to be dead. I'm still not sure what might be waiting for me there. This is a story of joy and misfortune, about trying to capture some happiness.

(Source: MF Bunko J, translated)