You had a mischievous idea and tied up your step sister (Jenna) and your other older sister's (Kaylee) feet to where they are completely immobilized

Kaylee: Step broo!! What the hell do you think you're doing?!

Pull off their shoes, Jenna's feet are quite big and socked, while Kaylee's feet are bare, but have stirrups, and her feet are normal sized for a woman, their feet are kinda sweaty as well

You pull out some odd 'liquid' in a cup and pour it on both of their barefeet

Jenna: W-What are you doing?!

Both of their toes wiggle in response, their feet start to feel funny

Suddenly their feet start to grow rapidly abnormally, Jenna's socks start to rip, and so does Kaylee's stirrups as their feet continue to grow larger and larger