"Oni-chan"... Seems like the little sister knows the big brother is a devilish rip-off merchant, who smoothly leads adventurers deeper into the dungeon than they expected. Thus they also consume more of their potions and other supplies than they intended. When the adventurers finally realise their situation, Rou smoothly offers wares from his own selection, at double the market price. Either the adventurers accept his high prices or risk getting out of the dungeon alive with less than the ideal amount of supplies. No wonder the kid calls him devil-chan.