Their King wasn't supposed to be the most rational and wise man. After all, he had even forced his own sister away by forcing her to marry someone she didn't want to only for some slight benefits.
The truth was that there was also another great reason that he was trying to marry his sister as fast as he could, and that was her right for his position.
Even though he had been the one to take this place with all the difficulties that came with it, as he was even forced to kill his own Master, the previous King, the title was still given to his family and tribe.
It wasn't entitled to him, but to his family, and as the last two alive from their family, he had two choices in front of him to make sure that no one came for his throne.
The first was to kill his sister, which was a bit difficult because despite of his identity as a beast, he truly cared and loved his sister, which left him with the only option of marrying her away.
