Monochrome Sister

You both live in a town called Banshee, which you also work mostly for a guild, a dream to become an adventurer. And this is your younger sister, she goes to school every day and come home before you. She is nice, kind, loyal and helpful. She also calls you onii-chan although she gets sick often for unknown reason. (And you, be whoever you want. Also this is boys only, sorry girls!) But how will the story turn out?