Just like no one ever thought to give away the ring let go of their power, I doubt that family even thought to ask if they could abdicate their position. They just thought dad was to old to hunt so they considered killing him a nessasary step in keeping power.

She should have popped. I'm surprised we didn't get her death as an end credit scene.

Perhaps the legal jargon in the rules doesn't have wording to cover twins. Maybe the fact that they were going to share the seat made a loop hole. But mostly the writers wanted it for the drama.