Level 1 kara Hajimaru Shoukan Musou: Ore dake Tsukaeru Ura Dungeon de, Subete no Tenseisha wo Bucchigiru

After losing his father in a tragic accident, Shinobu Iijima and his younger sister Megumi are taken in by their father's old friend, Ayumu. Shinobu works hard in the hopes of starting a new life for himself. Megumi, and a mobile game called La Vita Online, are the only things keeping him afloat. One day, Ayumu reveals his true intentions of tearing Shinobu down and getting his hands on Megumi. But just after Shinobu learns that Ayumu deleted all of his game data in La Vita, their high-rise apartment catches on fire and Shinobu falls to his death, Megumi in tow. He wakes up in a strange new world... and realizes he's been reincarnated into the world of La Vita Online. An announcement from the devs tells him that Ayumu is already there, and a countdown to Megumi's arrival is ticking. Shinobu vows to protect Megumi after she arrives at any cost, even if it means he has to crawl his way up from level 1 to fend off all the other reincarnated players.

(Source: J-Novel Club)