Over the weekend David's parents left him alone with his older sister Marie, who was in charge of babysitting him, since she had a few years on him and was already 15 years old, while he was only eleven. Just like their parents the siblings were both river otters with silky gray fur.
They lived in the urban part of an average city where their parents worked for the same company, where they have met and at this rate will both die they often joked, but this weekend they managed to get some time of and decided to go on a vacation for the two of them, leaving their children with the necessities they needed until they returned.
Both David and Marie tried their hardest to argue with their parents, Marie trying to convince them to take David with them, while David wanted to be left alone, not caring where Marie went. But since David had made a mess of the house the last time he was left alone and he was still rather young, there was no changing their minds.
So when the day came Marie stormed to her room upset and angry at the world. She had made plans to stay the weekend with her new boyfriend, despite her parents being against it, but they found out and informed the parents of her lover. So now she was stuck home alone babysitting her bratty baby brother, who was already unhappy about the situation himself, so she was sure he'd do something stupid sooner or later, for which she would be held responsible.
Like his older sister David had made plans beforehand, to avoid being stuck being babysat, but his plans were of a quite different nature. Having discovered his way into the dark Web, through the help and guidance of "that one" classmate who was always busy with strange things, David had acquired a drug that promised to "turn the table" in this situation.
After arrival he stored the small pill in his room carefully hidden and stashed away from his parents eyes in a small box hidden under his bed. So when it was time for dinner, announced by the ringing of the doorbell, as his sister had ordered Pizza with some of the money their parents had left her with, he quickly crawled underneath his bed and retrieved the special little pill from underneath.
As he was heading downstairs his hand clasped tightly around the small object inside his hand, scared of what would happen if he were to be caught right now. In reality the pill could have been mistaken for any household medicine being about the size of an average painkiller and very similarly shaped, but he was still young and didn't know that or at least didn't think of it.
So as he sat down at the dinner table his heart was pounding inside his chest and he had to concentrate to stop his body from shaking. David felt like his heart had stopped when he saw his sister watching him, thinking he must have been caught and preparing for the interrogation. But to his relief she simply wanted to know what he wanted to drink.
"Oh may I have some coke sis? Please! I will even get it myself."
He was so tense even his mouth felt dry from fright, but at least his thoughts were clear enough to make out a plan in his head.
"Sure but get me a glass as well will you?"
His poor exhausted heart skipped a beat as he felt like he had won, as his plan was working as intended. He quickly got up and went to pour them both a glass. While his sister was still setting the table, he glanced over making sure she wasn't looking. When he was sure she wouldn't look he slipped the pill into one of the drinks, making it fizz a little more as the small pill quickly dissolved. He made sure he waited for it to stop before he made his way back to the table and paid close attention to give his sister the one he had spiked.


As she was taking sips from her glass David kept a close eye on her,  waiting for the drugs to set in. As Marie noticed how her brother kept staring at her, she returned him a few looks of anonyment, wondering what trouble he had planned. But David didn't care to play it carefully anymore, his sister had taken the drugs and with time he started to notice the little changes happening to her body. 
What the young boy had slipped into his sister's drink was an illegal and very special youth pill, usually used to keep wrinkles away, but in its concentrated form it was said to actually reduce someone's age both physically and mentally. Which was the reason why it was outlawed, making it only available through shady means like the dark web.
So he made sure to notice how she slowly began to shrink, while both her facial and other features became young again. Especially her boobs were noticeably shrinking, until they were flat again. But she didn't notice all of that yet as she was focused on eating and of course chatting with her new boyfriend. Only when she had to get up the changes were too drastic for her to miss, having shrunk so much her pants fell down to her feet the moment her feet touched the ground. 
She shrieked in horror as she realized what had happened to her, in fact it was still happening, as David had used the wrong dosage. Quickly Marie realized that there was only one person in this room who could be at fault for this mess, but when her angry gaze met her brother, it quickly turned to horror as she saw him staring at her with a lustful gaze and a big erection pointing at her. She screamed at him but stopped after she heard her now high pitched childish voice. 
She tried to think of a way to escape, but her mind began to get hazy as she grew too young to gather any clear thoughts and when she talked only the brabbling of a toddler came out. David who had freed his painful erection walked over to pick her up and taunt her.
"Now who needs a babysitter now huh? You do and not me now let your big bro take care of you~"
He didn't think he was a pedo, but when he had looked at his sister's tiny and naked body, when her pants fell down and his gaze met her tiny toddler pussy he felt his dick grow incredibly hard inside his pants. He hasn't seen a pussy before now, but the boys in his class had started to talk about sex a lot and how good it was, though being the same age as him, they probably havent got a good look at a girls nethers themselves.
So he at least had heard how it was done and with his sister being quite literally as helpless as a toddler and his pulsing cock begging to take her there was not a second thought. He hastily took her upstairs and to his room. Once there David put the toddler on his bed before he quickly took off the rest of his pants. While all that happened Marie wasn't able to understand what was going around her, having forgotten anything about sex, but still the look on her brother's face frightened her.
His rod was an impressive size for his young age, likely to grow into an even more monstrous tool once he grew up and reached puberty. Especially when he compared it to a toddler, it looked frighteningly big, able to tear the girl appart with ease, so he was sure to make good use of it. However unexperienced as he was, he simply skipped over any preparation and just began pushing his sizable cock against his sister's tiny cunt, being frustrated as it didn't seem to go inside.
She immediately began screaming out in pain as she was neither lubed nor big enough to fit a tool like his inside her, but that wouldn't stop the lusting preteen on top of her. With enough force and some pre he leaked, he eventually managed to push his tip inside of the toddler, her walls trembling and clenching incredibly tight around it as he tried to thrust in even further. It was hard work as she was still dry and way too small for him, but he kept forcing it further and further, taking her virginity with brutal force and eventually making it all the way inside of her.
Since it was also his first time, his movements were clumsy, hurting his sister even more than they had to. But he was a quick learner, so eventually figured it out and was able to thrust more quickly and harder inside of her tiny tunnel. Being a virgin he was unable to last very long, so after just a short amount of time violating his tiny sister, his first load began pouring inside of her womb, while his cock was throbbing aggressively, stuffing more  and more of his seed deep inside the toddler.


After it was over he suddenly collapsed on top of her, never having had an orgasm before the sensation proved too much for the young boy to handle. Even having finished its mission, his dick was still hard, so with the additional weight it forced itself even deeper pushing beyond the cervix and inside the immature womb. Unable to hold himself up, the weight of his body almost suffocated the tiny toddler underneath him. But luckily for him and maybe not so lucky for her he noticed her desperate breathing just in time. So he gathered his strength and rolled on his side to not break his new toy. 
However at this point he slowly began to realize what had just transpired and what he did, even if his parents weren't to find out about the sex, he was a big heap of problems. David had irreversibly turned his sister into a tiny toddler and they were sure to notice their daughter missing if he'd try to hide her. So without any other means to solve the issue himself, he went back where the truble began and started to search on the internet for answers, he quickly learned more about sex, especially what had just transpired. At first he felt like a knife had punched through his guts when he understood how wrong it was what he had done, but somehow when he looked back at his sister, he felt his cock rising again, now wanting it even more badly than before and fortunately for him he had left until Sunday before his parents returned home. 
So he was sure to make the most of it, he looked at his still exposed sister, his cum leaking from her gaping and abused hole, while she was sobbing quietly on his bed. Since he couldn't make it worse, he thought to himself as his cock ached for more. The toddler recoiled and screamed as he touched her again, but he didnt stop, pinning her down and beginning the procedure all over again. 


This time however it somehow felt different to rape his sister, now he was more aware of how forbidden it was and how much pain he put her through. He thought it would stop him at some point, but truth be told the more he thought about it, the better it began to feel for the young boy. He wasn't doing stupid stuff out of curiosity now, he was raping his toddler sister and he knew in how much pain she must be and he wanted her to hurt more.
So he began to thrust even harder inside of her tiny entrance, his throbbing cock pulsing hard as it punched through her cervix and into her immature womb. The toddler underneath the boy, had no strength left and her screams were silenced, as the sheer force of his cock pushed the air out from her lungs and she had to desperately grasp for air. 
David relished in his sister's pain, even slapping her face harshly when he felt like she wasn't showing it enough. Something grim and dark had awakened inside the boy as he almost fucked the life force out of the young child, wraped with her tight walls around his cock. During all of this he remembered something he had forgotten until now.
When the pills arrived he had ordered there was another different one with a letter attached to it, showing the cartoonish image of a large cock. He didn't know what to do with it until just now. So with his dick still stuck inside his sister he managed to pull it out from underneath the bed and took it. Momentarily he felt his cock getting hot and his sister getting even tighter around him. When he looked down he saw the outline of his cock inside of her growing bigger and bigger, soon it reached all the way up to her throat.
Eventually he could even see his tip emerging from her mouth, how it had managed to do so without harming her he had no clue, but at least the look in her eyes told of how much pain she was in. So he picked her up, wrapped around his cock like a toy, and began pushing and pulling her up and down his still growing cock, slowly spreading her more and more around it.
When he felt himself getting close again, he made sure he wouldn't waste his load, so he pulled back and held her mouth shut, when he pushed back inside. His cock was forced to take another way and forced her belly to expand comically around it when she was used like a condom. Then he finally started blasting his massive load inside her.


This went on for the entire weekend, so when his parents returned home and went to greet him inside his bedroom, they were met with the disturbing sight of their son violating a toddler, which looked much like their daughter.