Well, not the way your sister did it, and never into a Kimberwicke. But I have transitioned to curb bits for years when I was showing and they were required. That said, the transition allowed for lighter rein aids, less noise and pressure in my horse's mouths, and was in no way harmful or irresponsible. There was good reason for the transition, and even benefit to the horse.
But, long before being shown in a leverage bit, my horses were fully trained from ground work to bosals to snaffles and were completely responsive to body aids and neck reining before they were transitioned to the curb. The transition is started with two sets of reins in a mullen mouth Pelham to educate the horse to accept a solid mouthpiece using snaffle reins, then adapting the horse to responding to signal from the shanks with the curb reins. Last, the horse is finished into a solid mouth curb.
The curb bits I use have enough shank length to provide ample advance signal so leverage force never has to be engaged. After a couple times of engaging the leverage during training, the horse quickly learns to respond before it is engaged, as soon as he feels the vibrations through the shanks. A Kimbrewicke makes that kind of advance signal impossible, and is not a bit i'd ever use for that reason.
The double bridle is necessary for advanced collected work in dressage requiring the curb action to signal for flexion and suspension. The snaffle reins are used for all other movement requiring direct contact.
The only reason a western horse can be finished into a curb without need for snaffle reins is because he is taught to respond to the neck rein for all lateral signal and to function on loose reins. Shanked bits can't be used for direct lateral signals or with continuous contact.
So, I disagree with anyone making a sudden transition just to enter a show, unless it is to remove the bit and use a Dr. Cook bitless bridle. Even then, though, it will confuse a horse already being stressed by just being at a show.