Jon fired. He suspected something like this might happen. He knew Sarah couldn't be trusted, but he thought he'd give her one honest chance to do the right thing.
Even as he was asking for the stone, he was also aiming the gun at Sarah, staring intently at her mouth, or more specifically, her tongue. When Sarah started to make her wish, Jon fired.
The gun still in growth mode fired at Sarah's tongue, causing it to swell, grow and fill out her entire mouth. When Jon was done, Sarah's tongue was so large, it kept her mouth open permanently, and stuck out about an inch. It was probably difficult for her to breath through her mouth, and it would definately be impossible for her to eat anything, but Jon didn't expect to leave her like this for long.
"MMWHA MHWAA!" Sarah cried out words that no one but herself could understand. Her hands flew to her mouth and she turned back to the mirror. Her eyes showed looks of terror as she realized that yet another thing had gone wrong with her. She looked at the stone and yelled incomprehensible wishes at it. Nothing happened. It didn't glow. It didn't grow hot. There was no flash of light.
"Sarah," said Jon calmly with an outstretched hand "give me the stone"
Tears covered Sarah's face and she mumbled something that Jon could only guess was some sort of profanity.
"Sarah, give me the stone, and I promise I will fix you the best I can."
She looked at him with hate and distrust in her eyes.
"...but, if you don't give me the stone, I'll have no reason not to keep going..." He pointed the ray gun at her with his other hand.
Sarah finally realized she had lost. Jon was in control now. Sarah had no way of fighting against him. Her only power over him, the stone, was now impossible for her to use. Dejected, Sarah finally gives in and hands Jon the stone.
"Thank you Sarah" Said her identical twin.
