Erin reached to snatch the flask of growth serum in Joe's hand... but withdrew at the last second. Sure, it worked, but they didn't know how well it would work on an adult. Besides that, she loved Joe, and couldn't bring herself to just snatch it from him. The scientist in her also reasoned it'd be wise to observe the full effects on the entirely unintentional test subject that was Joe's sister Megan first. For all she knew, the oversized and oversexed 7 year old could collapse into her original size any minute, if not smaller.
So with those thoughts in hand, and a healthy dose of genuine concern, Erin ascended the staircase with Joe. Thankfully, the door opened with no problems, and the area around it on the other side seemed to be relatively intact. Once they were out of the basement, it became clear that most of the damage was limited to where Megan had fallen through, yet they both knew far more damage was to come before this was over. The young giantess wouldn't sit there forever.
Joe rounded the corner and opened his mouth to scold his sister, but lost all train of thought the moment he laid eyes on her. The cute face of his 7 year old sister was gone, replaced by that of a young woman modeling agencies would trip over themselves to hire. It was like Megan at 21 if she hit the genetic jackpot, then grown and magnified further. "Megan! Are you alright?" he asked, trying to focus on something other than the awkwardness of being painfully hard for his sister.
Megan hadn't quite known what the blue liquid was in the flask earlier. She just assumed it was Kool-Aid, so she had taken a flask and drank its contents while watching her show. She barely noticed anything was off when she let her brother's girlfriend Erin in, but really noticed when her head hit the ceiling and her clothes were tearing off. It was hard not to notice the growing boobs on her chest, and laughed with delight as her body continued to expand... and eventually fall through the floor.
Now, she felt guilty as she was faced with her brother, knowing she was in trouble, yet didn't feel as guilty as she probably should. She was a giantess, and she loved it! She was bigger than any of the kids at school, bigger than her favorite character GinormaGirl, in more ways than one, and she knew Joe loved it too. Still, if she'd known what she was drinking, she would have at least gone outside to spare the house. "I'm okay, Joe..." she replied, "I'm in trouble, aren't I?"
Joe couldn't help but spy Megan's teeth as she smiled nervously. Before, there had been some gaps where her baby teeth had fallen out, as could be expected with any 7 year old, but now had an immaculate set of adult teeth that belonged in a toothpaste commercial. He didn't understand how this was possible. "Well, all my equipment and materials are ruined under your butt, you broke the floor, which insurance will never cover, and clearly stole one of the serum batches I was testing... Yeah, you're in big trouble."
Megan didn't argue. Like her brother, she was smart, and had known she was in trouble before she even asked. Still, her body felt so good, and she knew Joe liked the new her. However, what caught her attention was the mention of another growth serum batch. A part of her wanted to take that one too and see what kind of effect it'd have on her, and knew Joe was powerless to stop her, but another part knew it was intended for Erin. She liked Erin, but now felt... something more towards her, and wouldn't mind seeing her as a giantess too.
"L-let's focus on guiding Megan out of the house, first." Erin suggested, "She can't stay there."
Joe agreed, but getting his now 30 foot bombshell of a sister out would be tricky. The floor was already severely compromised, but the kitchen floor had reinforced joists from the heavy tiles that were recently removed. If Megan was careful, it might support her weight, and she might be able to get out through the large sliding door. "Megan, can you climb up onto the kitchen floor?" he asked.
Megan glanced towards the floor and understood the plan. "I'll try." She slowly rose from the hole in the floor, unintentionally widening it as her breasts and ass smashed their way through. She carefully walked forward, back pressed against the ceiling, and then gently lowered her upper body on top of the kitchen floor. It groaned in protest, but still held. She managed to get her head through the sliding door, and her shoulders, but her hips and ass finished the destruction her breasts had started when they went through.
Nevertheless, she was finally free after the half hour of crawling, squirming, and squeezing needed to minimize further damage to the house that was now too small for her. Relieved, Megan stretched, and stretched, and stretched to her full height, letting her entire body be bathed in the morning light. "I wonder if this is how a butterfly feels after it emerges from its cocoon?" she wondered absentmindedly, relishing the feeling of the warm light and cool breeze against her bare skin.
Joe and Erin came outside and beheld Megan examining herself. It was one thing to see her indoors, but seeing her outside in her full, gigantic glory was another entirely. Her legs were proportionately long. Her hips and ass flared beautifully, complementing her solid thighs. Moving up past her slightly muscular stomach, the huge teardrop shape orbs seemed to impress even her with their size and firmness. The arms and hands cupping them were slim, but not scrawny, and her face needed no introduction. She was easily the hottest woman either of them had ever seen.
Joe believed he should refrigerate the remaining serum, and keep it from his hotter than had any right to be sister. However, he's quickly stopped by Megan's huge, definitely adult hands lifting him into the air like a toy. The touch alone was enough to make him cream his pants, having been already unbelievably aroused by her new figure and size despite constantly reminding himself she's his sister. "Megan? W-what are you doing?" he managed to squeak.
"I know all about your giantess fetish, Joe." Megan replied, holding her brother against her chest, "I've seen some of the things you have in your favorites on your smartphone. I know you like this a lot..."
Joe blushed. Did she know how sensually she spoke just now? "A-are you trying to blackmail me to let you off for what you've done?"
His world shook as Megan laughed. "No! I want to thank you for helping me know why GinormaGirl's my hero! Watching her on TV has always given me a tingly feeling, and the stuff on your phone gave me that same feeling." she said, "I didn't know you were working on making that real, I thought that serum was just blue Kool-Aid!"
"Then why would I put it in a flask downstairs?"
"To use with Erin for pretending to be giant, I guess? It's not like you would've told me what it was, not even thinking I might like to be a giantess, too..." Megan replied, "I'm so sorry about what I've done, but," she paused, moaning approvingly as she freed a hand and gave one of her huge breasts a squeeze, "Mm, I love the results! My boobs are so big and sensitive... I never want to be small again!"
Erin stared dumbfounded the whole time, desperately trying to keep her hands out of her panties. She couldn't help but feel envious. She should be the sexy giantess, not her boyfriend's little sister. Joe should be happily exploring her enlarged body, especially her newly grown breasts, not held against Megan's generously endowed chest! She also feared she'd let the chance to live her fantasy that had been dangled before her slip away, possibly the only one she'd ever have.
Still, Erin noted that Megan's behavior wasn't quite lining up with how she behaved before the massive growth. Her speech seemed more intelligent, mature. And she definitely didn't hesitate to touch certain areas of her anatomy. Perhaps the serum matured her mentally as well as physically? Or maybe she's always been this way, and has now dropped the act?
Megan's announcement both aroused and scared Joe. Did she want the remaining serum? He tried not to imagine how big she'd grow with it. "Now, about that serum... I don't think it's a good idea to stick it inside, if that's what you were going to do."
"But it needs to be refrigerated!"
"Uh, I drank the other one at room temperature, and grew huge, bro." Megan countered, "I don't think it needs to be cold."
Megan was about to point out how it also risked someone else drinking it by mistake when she felt something wash over her. While she wasn't sure what it was, she silently hoped it meant she was about to grow again...
1) Megan grows again, but her proportions don't change! - It seems the serum may have a side effect of continued growth spurts after the initial growth. But Joe also notices that she seems more mature...
2) Megan grows again, but her proportions are altered! - The serum wasn't intended for a 7 year old. So as she grows, her proportions might not keep up... or might overcompensate!
3) Megan... shrinks?! - The serum wears off. Megan shrinks, but only to 10 feet with curves to match. She and Joe go with Erin to the local university's bio lab to perfect the serum.