Growing up, Molly had been a pretty good sister. The two of you weren't very close, but you never fought too much either. Sure, there was always teasing but you kept it lighthearted and nothing beyond what would be considered normal between siblings. You were just a year older than her, but even though you were close in age you never really ran in the same friend groups. Not to mention the difference between your personalities. An outsider might describe you as a jock - you played for the soccer and basketball teams and were fairly tall (6'1''). But the truth was, you weren't a meathead like some of the other guys on the team. You were pretty interested in science and didn't really enjoy going to parties or socializing much outside of school.
Molly, on the other hand, would have killed to be as attractive as you. She was always on the shorter side, only 5'2'', and unfortunately looked like she was still 12 years old when she turned 18. She was flatchested and very scrawny. It was evidently something she was very insecure of because you noticed whenever you were out in public, she wore baggy sweaters. Unlike you, the introvert, Molly wanted to be the center of attention. She envied the popular crowd who partied every weekend and had lost their virginities at 15. Molly wanted to be like them and she blamed most of it on her physical appearance.
It was unusual that Molly was so underdeveloped. Your mom, Angela, was 5'6'' and had C cup breasts. Your dad was 5'10 and had a stocky build himself. Your parents had taken Molly to doctors throughout her teenage years to see if her growth was stunted, but they all gave the same reply: "Some girls get the short end of the genetic lottery."
That's why when Molly's class took a field trip to a local chemical factory her junior year (your senior year) of high school, she was intrigued by the company's name: GrowTech. Molly never had the best grades and was abysmal at STEM classes, but she understood enough to realize that GrowTech made pills designed to help people, well, grow bigger.
Now obviously, this was not released to the general market, or it would be a billion dollar company. Right now their products were in testing stages and primarily used on livestock. Eventually, they would be used in third world countries to help malnourished children.GrowTech never planned for an 18 year old girl to steal a batch of their pills for her own personal use. Molly was desperate, though. Desperate enough to steal experimental drugs if it meant she'd have a chance at being beautiful. She didn't even really know if they worked, she just heard the word "growth" and took her chance.
All of this was unbeknownst to you, of course. It was unbeknownst to everyone except Molly, who returned home from that class field trip, giddy. She locked herself in her room and took her first pills. She'd stolen two kinds: General Growth, which promised growth throughout the body, as well as Breast Growth, designed to boost milk production on dairy farms. Molly swallowed one of each, eager to see the effects set in. And this is where our story begins, the story of a growing sister and you, her brother.