Joe began to feel an increasing warmth emanating from his sister's body just moments after she'd fallen silent. Given that Megan currently held him against her chest, between the fleshy mountains of her breasts, he was quickly growing uncomfortable. Furthermore, while she might have a point as to whether or not the growth serum he created needed refrigeration, the heat her body was throwing off could negatively effect what was left of it. But more than that, he worried for Megan's health.
Megan herself was also worried. She felt like she was burning up inside! Yet before she could imagine the worst, the sensation shifted to a burning of a different kind. It felt like it was coming from her crotch. Her hands had come close to touching there before, not knowing what she's supposed to do. But now, upon letting a finger enter her womanhood for the first time, she knew exactly what her body wanted, and was all too happy to oblige. A second finger joined the first while she tried to remain standing.
Joe couldn't see past his sister's hand from his position, but Erin could. Seeing the over 30 foot Megan shamelessly masturbating erased all resistance to do the same Erin had, and her hand finally found its way into her already thoroughly soaked panties. What she saw happen next only served to quicken the pace at which she was pleasuring herself.
Joe noticed what at first seemed like Megan adjusting the hand holding him, but upon seeing her rock hard nipples growing further away, he realized that she was growing again. But he knew she couldn't possibly have had any more of the serum... which meant she didn't need any more to keep growing. While definitely arousing, this wasn't an intended effect, as he had wanted at least some control over Erin's growth. Like this, there was no telling when or even if Megan would stop growing!
Megan moaned lustfully, the feeling of her growth and pleasuring herself positively intoxicating. Better yet, as she was outside, she didn't have to worry about running out of room, and so was free to enjoy feeling her limbs stretch. The grass tickled the soles of her expanding feet as she observed her house, which before she could rest her breasts on its roof while standing, become smaller and smaller to her perspective. "Oh yes..." she breathed, her heart beating like a jackhammer.
Yet even through the dense fog of her arousal, she hadn't forgotten her brother. She loved him with all her heart, and looked up to him even now, if just figuratively speaking. Her growing hand shifted to now gently cradle him, allowing him the freedom to at least turn around and look at her swelling face through her cleavage as the wind played with her dark brown hair. And though she recognized he'd be safer on the ground, she figured the least she could do for him was to let him enjoy being with a giantess as she grew even more gigantic.
Since Megan's growth began, Joe came more times than he could count. All he knew was it was far more often than any brother should with his sister. Though he had noticed she was acting way too mature for a 7 year old... Regardless, he hadn't time to dwell upon it as he felt a sudden, gut-wrenching drop, which he realized was her merely sitting down. It blew his mind that such a casual act as sitting would feel like an elevator suddenly giving out beneath him, she was so big... and still getting bigger!
Soon after, Megan felt a pressure building between her legs. She barely managed to turn to face the nearby woods before exploding with the first orgasm of her life, cum soaking all in its path. As she basked in the afterglow, she realized she'd stopped growing. While initially a bit disappointed, something in the back of her mind told her this had been but the first of many spurts. She was thrilled that her proportions had remained unchanged, keeping the figure she'd fallen in love with.
She very nearly had a second orgasm when she discovered just how much she'd grown. She was so big, she could give her previous height a boob hat - while sitting. If she were to stand, Erin could stand on the roof of the house and still fall just shy of reaching her knee! "I... I must have tripled my height... That would make me," Megan paused as she calculated, "well over 90 feet tall. Perhaps just shy of 100 feet! Holy cow I'm huge!" she cheered. She loved being a giantess!
As soon as Megan set him down, Joe darted indoors. His sister's orgasmic scream alerted him to the need for a safer container, and while not very scientific, a clean thermos would suffice. He just hoped this serum worked as well as the other, though there was no way he could let Erin drink it yet. A part of him wondered just how huge Megan would be before he's ready for his girlfriend to even try to catch up to her... He doubted this was the last time she'd grow.
First though, he needed to synthesize more of what he had now. His lab was ruined, and even if it wasn't Megan's orgasmic scream would likely cause the entire neighborhood to come looking, so he needed somewhere else to work. Luckily, Erin had offered the use of her campus' bio lab, which may even have what he needed to recreate the serum. Once that was done, he could work to perfect it. He could possibly create something to keep Megan from growing further, but there's no way he could make her take it if she didn't want to.
Coming back outside, Joe found himself in Megan's shadow, her colossal form crouched over the yard. Looking up into her face, he smiled, and was rewarded with a powerful gust of warm, humid, surprisingly sweet smelling air. He couldn't think of her as his little or younger sister anymore; her actions just too mature and consistent for that. "Megan? I don't know how to say this, but... you seem different, and not just physically. Have you noticed anything different in your behavior since drinking my serum?"
Megan thought it over, before nodding. "Not at first, but... I think during my time of merely being over 30 feet, my mind was slowly changing. My recent growth must have sped it up to how I am now." she answered, "If I had to guess, the sudden change in my body physically forced a mental change, albeit delayed. I can remember some of the big words you've said when explaining things to me in the past, and actually understand what they mean, among... other things no child should."
"Joe said the formula seemingly causing a very rapid and extreme puberty in a girl who hadn't gone through it yet in addition to the growth, so..." Erin mentioned as she came up beside Joe, her gaze transfixed on the giantess' heaving bosom she wished was on her flat chest.
Megan's face lit up with understanding. "So I basically went through the fastest, most brutal puberty the world's ever seen! The physical part finished rapidly, before I even broke through the floor, but the mental part only just recently finished! In other words, I act different because I'm mentally as old as I physically look now, definitely out of the teenage years. I'd say 21? That sounds about right." she concluded, adding with a snicker, "I doubt it'll have the same effect on you, Erin. I don't think my bro's into giant grannies!"
"Speaking of giants, Joe..." Erin inquired, putting on her best puppy dog eyes, "When am I going to get to grow into a giantess like your sister? Wouldn't you like to see knockers like hers on me, towering over you and maybe letting you reach up to touch them?"
"Er, I... don't think you'll ever get as busty as me, Erin." Megan interjected, "I might've been short for my age before, but I doubt these beauties are entirely the serum's doing. I was likely due for about C-cups naturally..."
"Uh, not for awhile. Yes the serum works, but I didn't intend for continued growth after the dose. Frankly I'm amazed Megan can even move under her own weight at her present size, let alone so easily. If her growth continues, she may run out of air to breathe and die. I don't want to take that chance with either of you." Joe replied, trying to get away from Erin and Megan talking about breasts. "So, uh, if I could use the equipment in the bio lab at your university, I'd appreciate it."
"For you, Joe, and for the chance at living my dream, gladly." Erin replied, "I'll even help you with it if you like. It should go faster that way than doing it by yourself. Though it is a fair drive from here, and the bus to campus doesn't come by here..."
"I'll carry you two there. I'm big enough that I could just walk straight there and we'd be at your university faster. Granted, I also need to watch where I step..." Megan offered.
"But people will see you!"
"So? It's part of the territory of being a giantess! Don't tell me you've never fantasized having dozens of eyes, if not more, upon your towering form, including Joe's." Megan countered, "Besides, It's better they see I'm a gentle giantess before I'm as big as a skyscraper."
"Actually, that's a good idea." Joe said, "And I want to run some tests on you, Megan. Maybe if I can see why you grew again, I can come up with a counter for it before it's too late. I promise I won't try to reduce you... though mom and dad are probably going to kill us both for wrecking our home and making you no longer "mommy's little girl" but "mommy's gigantic sexpot daughter"..."
"Worth it in the end. Granted, dad might be too distracted by my giant boobs to punish... and maybe mom will let us off easy if we offer her a chance to grow big, too? Perhaps a variant that can rejuvenate her to her prime if it can't do that already?"
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