The My One-Hit Kill Sister.

  Asahi, a boy who loves video games is killed in a traffic accident and ends up in another world. He tries to enjoy the otherworldly adventuring life he's always dreamed of, but his stats are on the level of an NPC's. He is in big trouble and is being chased by a monster when he hears, "Fear not, little brother!" and the monster is killed in one hit by Asahi's older sister, Maya, who followed Asahi to this world. So Maya has acquired some seriously OP skills, but she takes her adoration of her younger brother to extremes. Thus begins their story about an overpowered older sister with a brother complex and a younger brother with the weakest of stats in another world.