"Why am I in these?? I'm eleven years old!"
"Just for protection," you smile at your sister, "drink up."
"Why do you have to be the boss when Mum and Dad aren't here?"
"Because I'm the oldest! And more mature than you!"
"Being 'mature' doesn't mean put me in diapers."
"You'll thank me later."
LATER
Little sister tries to open the bathroom door.. but it's locked. "Why is this locked?!" she shouts to her sister.
"Oh I don't knowww..."
"I'm actually going to pee myself."
"...What are you wearing?"
"No! I'm not going to pee in a diaper!"
The older sister pulls her over and starts tickling her, pushing her back on to the sofa. "Stop it!" she squeals, but she won't. She lies on the sofa, spreading her legs and the older sister puts their hand on the front of the diaper.
"Did someone have an accident?" she asks, gradually feeling their hand becoming soaked. Her sister nods, confirming the thought. "Do you want to be changed?" She shakes her head then goes onto the floor and grabs one of her toys. She puts it in between her legs, sits forward a little, then rocks backwards and forwards, not caring that her sister is watching.
Suddenly it's dinner time and she still hasn't gotten changed. In fact, around three fourths through it a large farting sound fills the back of her diaper.
"Does someone need to make a stinky?"
She stands then goes over to her sister, sitting on their lap. They rub her stomach until wet farts nearly soak the seat of her diaper, then she does one final big push so the poop lands in her diaper. The older sister feels a lump pushing harder between their legs.
The younger sister is relaxed and nearly out of it, so they grab the buzzy item and turn it on, putting it on and nearly in the front of her diaper, while they rub forwards and backwards, smushing the mess.
"Are you a good girl now?"
Soon, the front of the diaper will become damp again, and the younger sister will realise she enjoys this.
Mum and Dad come home, noticing their youngest in a soggy diaper crying and needing a change, while their oldest looks proud of themselves.
