Kevin felt strange. He felt the can he was holding becoming heavier and saw how everything was becoming bigger and bigger he was shrinking. He shrunked down till he was merely 3 inches tall 
"What happend to me why am i small? this cannot be real it must be a dream" 
Said Kevin. But it wasn't a dream it was real and when Emily couldn't belive her eyes when she saw the potion really had effect she walked towards her shrunken brother and gently grabbed him and lift him to the height of her face.
"Well well looks like i am the bigger sister now"
"Emily what have you done to me?"
"Nothing little brother. Let's just say we will have fun togheter"
Emily walked to her room and layed her brother on her bed
"What are you going to do to me Emily?" 
Said scared Kevin
"Well you always joked about my diapers. Let's see how you feel when you are put in one" 
Said Emily while taking a fresh diaper from the drawer 
"You want to put me in a diaper? well bad news i am too tiny now to fit one of your diaper"
Replied Kevin
"No i don't want to put you in diapers i want to put you into my diaper!"
Said emily with a evil grin while changing from her clothes to the diaper 
"WHAAT? no you won't do it you can't do it!" 
"Oh really?"
Said Emily while pulling him up from the bed. Kevin tried to fight back but the grip of her hand was too strong for his now tiny form
"No please put me down!"
Screamed Kevin while emily pulled her diaper down.
"Not a chance! you are going down into my diaper and you gonna enjoy it for the rest of the day"
Said Emily and dropped him in the front of her diaper right in front of her pussy then she pulled her diaper back up and put on her grey leggings and went to the living room and layed down on the couch.It was dark inside the diaper and Kevin was wrapped in an overpowering smell coming from the butthole of her sister. He tried to move to escape but the diaper kept him tight. The hours passed and Kevin had to get used to that prison. He was feeling so helpless trapped in her sister diaper. 
"uh oh"
said Emily feeling her tummy rumbling while letting rip a soft fart. Kevin was gagging from the smell
"sorry little bro i think i have to go really bad"
said giggling Emily
"Emily no please don't..."
Kevin voice was cut short as Emily stood up making him slip on the back of her diaper just under her butthole.
"prepare to drown in my shit Kevin"
said emily while spreading her legs and pushing out a massive log of shit. Kevin tried to escape to the growing mass of fecal matter his leg were already trapped into it soon his whole body was caught leaving only his head out.
"Emily help m..."
Shouted Kevin on a last desperate attempt while that growing monster covered all of his body.He were trapped now he was suffocating in her sister waste.
Emily laugh as she squished gently her diaper feeling the mess while his brother were struggling for oxygen, she really enjoed the sensation of power she had, how the simple act of taking a shit was like a force of nature for his brother pathetic form. Kevin started moving with all his might trying to get a breath of air, Emily wetted herself on the sensation of a tiny creature moving against her privates, making the situation even worse for kevin. Luckly this situation didn't lasted for too long since the lack of air caused Kevin to pass out.
Emily waited until there were no more movement in her nappy and then she decided that was the moment to pull her brother out of his prison. 
She cleaned  Kevin with a towel and laid him on the bed of her bedroom and waited for him to wake up.
Kevin woke up after about 10 minutes.
"It was a dream?"
thought Kevin but he was horrified as he saw her sister towering in front of him. He were still small it was not a dream.
"Good morning Kevin, have you slept well?"
Kevin saw Emily as she was still in her messy soggy diaper
"it's time to change my diaper"
said Emily.
"What? there's no way i can change your diaper i'm too small."
replied Kevin.
"If you don't change it you will be crushed beneath it"
Kevin was terrified at the idea of dying under a diaper, under her sister diaper!
Her sister laid on the bed as Kevin started trying to pull out her diaper, but it were all useless he was too tiny to move that diaper.
"Kevin if you don't move it right now you are just going to die"
Kevin pulled and pulled with all his forces but the diaper didn't move.
"I'm sorry Emily i can't move it PLEASE don't kill me! please i'm trying"
plead Kevin
"Fine!"
Exclaimed Emily while standing back up.
"Then try to run, if you manage to escape from me you are free but if i catch you you are dead."
said Emily while putting Kevin on the floor.
"Run Kevin, i'll count to ten then i will try to catch you 10.. 9.."
Emily started to count as Kevin started to run as fast as he can 
"5.."
Kevin felt the panic grow stronger as her sister was approaching to 0
"3..2..1 I'm coming for you"
Kevin kept running and running but it took her sister just a few step to get infront of him.
"I won!"
Said Emily
"please Emily don't do it please i will not make fun of you i will do anything"
"Too late lil bro"
Said Emily while sitting on his brother.Kevin tried to stop her sister but his arms were just knocked back by the weight of her sister padded butt.Emily was slowing adding more and more pressure on the tiny body of Kevin.
"Emily please noo Emily!!"
Kevin begged for his life as his body was slowly crushed under her sister messy diaper.Her sister just teased him
"You are not so strong now huh? try to stop me now!"
Said Emily as she started to hear crack and pop from the bones of his brother.Kevin screamed in pain as he felt his bones braking under the tons but her sister just laughed as she released all of his weight on what he now consider a pathetic bug.
The body of Kevin were crushed into nothing as her sister just grinded his butt and patted his diaper feeling the mess in her diaper while crushing effortlessly killing Kevin. 
After that Emily just enjoied her messy diaper for the rest of the day. No one would ever teased her again.