As it turned out, he had his choice made for him. Before he could react or think much of anything, Mandy raised the vial to her pert lips and took a teensy tiny little sip.
Max stared in total horror. He didn't see how much she had taken, but it sure looked like enough to bring her down to his size!
Which meant... uh-oh. His boner suddenly began to shrivel as he lost his excitement.
Far above, older sister Mandy put the cap back on the potion and put it down on the dresser again with a pleased smile.
"Mmmm!" She moaned. "That tasted really good! I-"
The tall blonde staggered slightly and her cheeks suddenly flushed rosy red. She let out a gasp, put her hand to her head, and bit her lip in sudden and powerful arousal.
From the carpet far below, Max watched in fascination and horror as he realized what was happening.
Mandy wavered on her feet slightly and let out another sharper gasp and then a moan as she began to lose size rather quickly. Her tight tank top began to loosen and sag. Her tight jeans began to wrinkle a little as she filled them less. The poor girl had no idea what was happening to her as she had to brace herself on the dresser as her legs went shaky and slid down the front moaning in a higher and higher voice by the second.
Max watched the display with amazement as his big sister shrank down. Already four foot nothing, her pants fell down her legs to the floor and he saw her panties hang off one thigh with an obvious damp spot in the front.
"Oh God..." Max winced, but he couldn't deny how sexy this was.
Now getting closer to half her size, poor Mandy began to fondle and caress herself. She was tiny, shrinking, and now inescapably horny. Max watched her shirt lower to her knees and gasped as his sis groped her crotch, a part of the front of her shirt getting wet from her juices. She began to massage her groin and make adorable squeaking noises as she kept getting smaller and smaller.
Max watched as her shirt grew too big and slipped off her tiny shoulders and stared in astonishment as his buxom naked big sister. She had... really 'grown' this last year, hadn't she? He had never ever seen her naked before. Now here she was moaning and shrinking while her hand eagerly pushed into her wet pussy. She was completely oblivious to the change, only aware of how good it felt and enjoying it.
Then, to his further cringing horror, he realized she was shrinking even smaller than he had. How much had she taken?! As she started to slow and then stopped, Max realized that his sister must only be as tall as his stomach now. Like 1 and a 1/2 inches?! She was so tiny!
As the initial pleasure began to ebb and fade, Mandy suddenly gained realization that something was very wrong. She stared at her surroundings in a stupor, and finally let out a terrified squealing scream as she realized what had happened to her.
"Omigod!" She squeaked, turning in all directions frantically as she came to grips with suddenly being absolutely TINY.