Story
Miku presents her younger sister, Karen, with the game "Another World Online" because Karen can't play like normal kids due to circumstances.
Synopsis via AniList (English).
Manga
Tamer Shimai no Mofumofu Haishin: Mujikaku ni Mofumofu wo Tsutetekuru ga Cheat Kyuu ni Kawaii no de Jiman Shimasu
Adventure