A sinister idea popped into your head, and you looked around until you found the right serum. As they say, revenge is a dish best served cold. You had found the panties serum. Buying the serum from off the shelf for $50, you hurried your way back home. Your sister was in the backyard at the moment, and you took the time to read the instructions.
“A sip lasts a day, a gulp is for a week, and all of it is for a month. Mix with drink and wait for the right moment.” It read on the tiny label. Popping the cap on the vial, you poured the entirety of it into a bottle of water, closed the cap, and set it inside the fridge. You hid upstairs, and when your sister returned from doing yard work in the backyard… she was hot and sweaty. She opened the fridge, saw a bottle of cool, crisp water, and gulped a mouthful before filling what she had drank with water and a raspberry tea powder, and returned it to the fridge. You began to laugh walking down the stairs, and your sister was confused.
“You see, I don't get mad, I get even. And I know mom was looking for her lucky panties for her "date" tonight with her girlfriend. She can't find them anywhere. You seem like the perfect replacement. Of course she won't know it is you.” You stated. Your sister raised her arms to her face, which were growing pink already. She flailed, her arms and legs becoming thinner and cotton. After a minute, she had become a pair of pink panties, which had fallen to the floor in a heap of clothing. You retrieved your pink sister from the pile and carried her up to mom's room.
“Best part of this, you can hear, feel, smell, and taste everything that is around you. And don't worry, the serum makes you super absorbent. Maybe this will teach you not to keep your brother as a bra.” you said, dropping your sister into the panty drawer.