You decide you have nothing better to do. You jog after your sister to her next class. On your way there you notice a few braver students giving cat calls her way, which you suppose would be pretty standard now with her new assets. You continue to run after her, and see her going through the gym doors. Oh my, her next class was P.E. This would be fun. When you get to the doors it occurs to you that you're still holding your sister's bra. You tuck that away in your coat pocket, saving it for a special occasion.
You follow her until she reaches the girl's locker room. Taking a deep breath you decide to test your boundaries and step inside the forbidden room. There is a strong permeating smell of perfume lingering throughout the locker room. You are assailed by images of budding assets of varying ages from 14 to 18. All of this would be enough to make a young boy drool, which you are. breasts large and small, butts in all forms. You attempt to recover yourself from getting too distracted, and attempt to find your sister. Her locker is in the far corner, lucky for her braless self. You saunter over to her, and stare as she tries to get on her undersized gym shirt, cursing quietly as she struggles.
She takes her pants off, and you see her butt nearly naked, save for some riskily thin panties. Outside of her usual clothing of tights, her butt leaves a bit to be desired. She really was flattering herself with her clothing choices. You decide that she could use a bit more junk in the trunk. You silently wish for her butt to start expanding until you want it to stop. You begin to see the flesh on her behind shifting. You move to get a better view from the side. You begin to see it visibly bulging outwards. After a few seconds it is by any measurement very large and still growing. You decide to slow the growth but not stop it, so that it would grow by one inch outward per minute. She grabs her gym shorts and attempts to put them on but failing due to the increasing size of her butt. She struggles red faced, tits jiggling, before,thanks to the power of elastic waist bands, she was able to barely fit her shorts around her growing posterior. You both walk out of the locker room as she makes some remark about them making the shorts too small.
After leaving the locker room you both check the whiteboard to see what she would be doing today. Volleyball. Ooh what fun. You'll have fun watching her attempt to keep up with a slowly growing bottom half weighing her down. By the time warm ups are over, you would say that each of her cheeks are easily big enough to fill a hand, and would soon be able to fill two. Everyone gathers around the nets and prepares to play though your sister seems to be having a tough time getting into it. You glance again and notice her shorts are now looking uncomfortably tight around the back. And a very amusing bit still was that she had no clue that this wasn't normal.
As you glance around the room you notice that your sister was not the only notable person in the room. This whole room was filled with perfectly viable people to change to your whim. For now you choose to divert your attention towards someone else in the room, but who?