\n\t\t\t\t\tEbonyRubberWolf some weeks back. Since I wanted to see him apply his toyification to someone less furry than usual, you'll have to click here to see the whole image. The story is mine, of course.Some notes follow the story, moved there for spoiler avoidance.
Nathan never knew if his sister understood what the result of the experiment would be. He was lying naked now, as instructed, on the soft, low bed, thinking about how she had looked just before the procedure started, her bright blue eyes meeting his, a nervous little smile on her pink lips, her straight, blonde hair touching her little butt.
The little monitor had turned on when the sensors were connected to her chest. Heart rate of 80, that was understandable given the situation, but why didn't it show breaths per minute? For some reason it was replaced by pO2 105, whatever that was.
Her rapid breaths certainly made for an interesting effect, Nathan thought. He hadn't seen her pantyless since she was a toddler, nor topless for a few years, but thinking of her naked now, still hairless but with such gorgeous breasts for a 13 year old, he felt himself stirring. And with a near adult's physiology a stirring quickly led to much more.
And now the next line on the display lit up, that was clearly Elapsed time, counting up from 0:00 -- and then she was there! His cock jumped in excitement: a naked body, round thighs, widening hips, a sex seeming to glisten already, slender waist and then those breasts. And nothing more. Her balance hesitant at first, she turned towards him, somehow sensing his location, and came closer, hips swaying.
If his sister's head had still been there, Nathan might have refused when she stepped onto the bed and straddled him, starting to squat down. But now it seemed he could only take hold of his erection and guide it between her plump outer lips, her sex wet and tight as it swallowed him.
Holding that slender waist, seeing the kneeling, young body riding on his shaft, he almost forgets the monitor, but after a while the changing digits catch his eye. Of course her heart must be pumping, he can feel how excited she is by the squeezing around his shaft, but the last two lines have appeared now. And as the pO2 number turns red, he makes the connection. A body that cannot breathe will not last forever.
But somehow that doesn't matter. Somehow, it only makes the sensations twice as intense, knowing it will be the only time. And as his thrusts get more urgent, they meet with a response. His baby sister's body above him tenses in what might be her first climax ever, and will probably be her last, as her heart rate soars and the red numbers plunge, the estimated times melting away, until she collapses forward onto his chest, the aftershocks of the orgasm clenching her sex hard around him.
Nathan never wonders how the brain functions that allowed his sister to walk and fuck without a head had been preserved, but they are clearly knocked out now, the sweaty little body lying flat on his belly and chest. As he continues to thrust, slower and deeper now, he watches the final estimate clocking down, feeling her muscles starting to twitch as the oxygen depletes, the desperate beats of her heart felt even through the cushioning of her breasts pressed against his skin.
And then everything seems to happen at once, her thighs hugging his waist tightly, her back arching, her softness clenching him hard, the heart rate on the monitor flickering and turning into a dash -- and Nathan groaning loudly, cumming hard inside his dying sister.
pO2, in case you're wondering, should really have a subscript 2 and stands for partial pressure of oxygen in the blood, approximate mmHg units as measured by a transmission pulse oximeter - for our purposes, 100 is a normal value, 70 start of brain damage, 50 unconsciousness, 25 death. For lack of fingertips and earlobes, the sensor is on her nipple...
Don't ask me for a more detailed background on this, like what sort of technology or even magic was used, how much of her brain it preserves and where, what happened to the other left-over parts, or why the Ethics Board hasn't nailed the experimenter's balls to the wall yet. It's clearly not 'our' world, is all I can tell you.
The same goes for why Nathan's sister becomes a test subject -- maybe he gets a cash reward (as well), maybe Nathan is a student at the lab and his sister volunteered because she thought it would get him a better grade... better suggestions are welcome!