Jack was absolutely delighted when he saw that he'd attained a form he'd thought he'd have to wait years to acquire, the aesthetic, bodybuilding form of Jeff Seid. He put up his arms in a flex and started going through some bodybuilder poses he had seen through his IG feed. Every single pose fed into the developing ego of the newly matured Jack. He felt at the scruff on his face and mused that he was now by far more handsome than the other guys in the house. None could equal his strength either...
The eighteen year old bodybuilder hurried out of his room and out to the bathroom, eager to really get to know his new body. He knew that he was making heavier footfalls than usual, but he no longer cared. If Will saw and freaked, it didn't honestly matter very much.
Jack was barely aware as he threw the door shut and locked it behind him, standing, shivering with excitement and eagerness, in front of the mirror. His skin was tanned like most bodybuilders had it, and everything was even better looking in the relief of the mirror. He turned and flexed his back, cooing with wonder as he saw how wide and strong his back was now. Nothing like the skinny athlete-in-training he had been before!
"Ohh yeah...wonder what my---" Jack wasted no time in pulling down his shorts, grinning when he saw his manhood, straining a pair of underwear that were basically painted on at this point. It was phenomenal, and he was immediately convinced he needed to ring in his new maturity with some fleshy indulgence. Off came the embattled boxers, which in fact ripped in the course of their descent down his ripped quads and calves.
Standing there with a ramrod erect manhood, Jack breathed for a moment...and then went to town on it.
With the combination of his immaturity for his new age and his eagerness from growing to such a size and aesthetic look, along with the sheer sexuality of his new body, it didn't take long for Jack to almost fall over from the intensity. His moans rippled through the house as he crudely jerked at his cock, no finesse, all sexual desperation and the anticipation of a man who had only just discovered the fruits of development.
His hair quickly became matted with sweat as his teeth grit tight, muscles flexing and flaring with life. His virility was on full display. "Ohh fuck...fuck! Fuck! I can't...I can't stop...I...Ooh! I'll cum...!" He thrashed about, knocking over some bottles of soap in the bathroom and the plunger in the course of his kicking, full force intensity. Nothing else mattered to eighteen year old Jack more than getting his rocks off. He was looking like Jeff Seid - and that demanded some attention to his needs.
And then just as quick as it felt like it started, it was over. Jack felt a white hot flash of sheer pleasure, something he had never really felt before, as he hit a thunderous orgasm right then and there. A singular force rippling through him as it sent him into shivers of delight, his body quivering, his mind reeling as it learned he could do this now - as his balls emptied themselves on the mat on the floor there. He pistoned his hips a few more times and a few more jerks made him gasp.
"H-H-Hey, wh-what the hell's...g-going on in there...?"
Jack was brought back to lucidity by the sound of a knock at the door followed by the typically timid tenor of his former older brother. Getting himself back up and not even bothering to put on clothes, he gave Will a look as he threw the door open, standing there with a confident sneer, "I grew up, LITTLE bro. Look at all this!" He flexed his arms and grinned with glee.
Somehow, Will recognized him, maybe it was just the hair color, maybe it was the behavior. Maybe it was something else. "...Jack? Is...is that you?"
"You know it brah. And far as I'm concerned, there's gonna be a few new rules around the house while Mom and Dad are out." Jack crossed his arms and leaned forward, "Let's start with I can cuss whenever the fuck I wanna cuss. I am eighteen now, and you're sixteen. So I'm the big brother."
"Shouldn't you be setting the example?" Will remarked offhandedly.
Jack responded by chuckling, "I am already setting a good example! That you should get as fuckin' ripped as me!"
Will just trembled in his place and realized by quieting down he basically acceded to this crazy, fucked up change to reality. He watched as Jack basically got even more into himself, flexing and posing and enjoying his new body. And it seemed, there was some aspect of mental maturity in place in some sense. At least it hadn't erased how things used to be.
Will shakily started to head into his room, and ignored his brother calling out and asking if he was alright. Will promptly fell unconscious into his bed.