You considered, for a moment upon seeing her, that you would make Apple Bloom an infant alongside her brother and sister, but two things stopped you from doing so: One, that you were already short on hooves to take care of the two babies you had and couldn't really manage a third, and two was that you didn't particularly enjoy turning someone her age into an infant. And besides, you always enjoyed the irony of the youngest sister suddenly thrust into the responsibility of caring for their "elders".
"Why hello there, Apple Bloom, is it?"
She could only respond by letting her jaw drop in shock at the sight of her two older siblings in a crib, wearing diapers.
"I'm glad you're here, Big Macintosh needs a change!"
Her pupils shrink. She turns toward you, finding your grin sickening. The slam of a door shocks Bloom into turning around to see that Granny Smith had snuck about her and closed her only route away from this situation. Her grandmother gives her an encouraging push forward.
"Go on dearie," Granny Smith says, "your little brother needs his diaper changed. A young mare like yourself has to know these things for when she has a foal."
"Wha- buh, buh, but I, I don't, he doesn't..." Apple Bloom stammers on as her grandmother slowly pushes her toward the playpen where her siblings eagerly await her. You unlatch the door and pull Applejack over to the side so she doesn't get in the way of Apple Bloom changing Mac. Little Applejack is such a hoof-full, you don't think you'd even bother having Apple Bloom try and change her. Mac is a much better place to start.
"Go ahead Apple Bloom," Granny Smith is still being as encouraging as she can to get Apple Bloom to move along. As she enters the playpen, Big Mac laughs a big, dumb laugh, clops his hooves together and falls on his back, presenting his diaper to her. She winces at the fresh scent of it.
"Well," she says, holding her nose, "what am I supposed to do with it?"
You laugh, "Oh Apple Bloom, you're supposed to change it! You know, open it up, pull it off, clean Mac's rear, powder him, put a new diaper on him and toss the old one! Easy as apple pie!"
She shoots you a sideways glance, "How in the hay am I supposed to do that? With my mouth?"
A smile dawns on your face. A smile that does the precise opposite of calming poor Bloom.
"Oh no," she protests, "no no no a hundred times no! There is no way that-"
You decide to cut her off by bonking her on the head with your hoof, "You help, or we throw you out. Besides, it's not so bad. Watch, I'll get you started."
You pop the straps of Big Mac's diaper off with your teeth, eliciting a giggle from the stupefied stallion. The bundle unfurls in front of Apple Bloom, opening up a bouquet of horrid aromas and sights that she never wished to see. She immediately brings a hoof to her mouth to stifle the wave of puke that flies up her throat the instant she was exposed to it.
"Go on Apple Bloom," you cheer, "you can do it!"
Granny Smith gives her a comforting pat on the back, "Yes dearie, go on and change your brother. It's as easy as pie!"
Between your and Granny Smith's encouragement, you manage to reduce the look of horrified disgust on her face to a look of sickened indecision, but will Apple Bloom be able to go through with it?