Responses
They might not want to give her power (of fame and glory) but still want to benefit from her work. Since the countess (FL's sister) is close with the emperor, she might've persuaded him to keep the FL's achievement hidden so that the FL wont be a threat in the power/political play. It's also likely that they sent her to slay the demon king in hopes that she would die during the mission, but she survived ┑( ̄Д  ̄)┍
Wait, she s like the hero warior of some prophetie that has win against some demon...were is the glory? Why isn t the queen/king doing the ceremony for her? Why isn t she famous ? And why do you let your sister, who is nothing compare to a demon, decide for you?
It s just doesn t seem realistic to me. Kind of like it s for the sake of the plot, but i don t get into it