Both of them were tricked into this.
The Church actually kidnapped the king a while ago, have one of their follower act as him (via a Witch's magic to turn him to look like the king...yes you read that right) and suicide like this to ensure the siblings can't just cooperate at all.
Their goal is to basically unite the kingdoms under their rule for a certain purpose.
MC crush them and do a much better job at the thing they were aiming for.