He should be dead after that, but I guess it was just a comedy manga special effect. That being said, it would be strategically wise to slay the brother (in self-defence) because he's her worst opponent what comes to assuming the leadership in the family. Outsiders would also gleefully use him against her, whenever possible and necessary. Worst of all, as long as he exists, there would always be a strong reason for outsiders to kill Lunaria, to automatically make that cheater the next head of the family. If he doesn't exist, nobody knows who would inherit the family, so less people would dream of doing it and benefitting from it.
Siblings always killed each other in inheritance struggles anyway, so I doubt anyone in the kingdom would as much as bat an eye at it.
