The rightful downfall has begun. I'm willing to bet the Elder sister can't act for shit and it was their brother that made her seem like a good actor. All the sisters are actually talentless.
The way it is done is far better. I think the way you wanted to be is vastly overused and done to many times in stories. This way is much better for Kumugi's character growth.
Brunhild is nuts. Instead of using a weapon of the gods as a amp, she used it to harden her own body to withstand her own amps. Also why if both weapons are suppose to be equal in power, why is one afraid of the other?