You suddenly begin to grow, your dress is ripped apart as you get taller and bigger. Looking down, you see two well developed breasts and soon realize that you've been turned into a 16 year old girl, but you're still wearing the same wet diaper.
You figure Sarah must have done something wrong, and run away before she can get you again. You head straight for your room and lock the door. Once in there, you take off the gross diaper and put on one of your old sets of underwear, even though it doesn't fit.
But just then, you feel odd, you have to go again! Before you can do anything, you pee all over your underwear and thoroughly soak it.
"Ahh, what happened? I couldn't even try to stop it!"
By then Sarah had managed to open the door and is watching you from across the room.
"What's wrong Bonnie? Still haven't mastered the potty yet? It couldn't have anything to do with me making you unable to change yourself unless your wearing a dirty diaper could it? And it certainly isn't because you will wet yourself instantly if you're ever wearing something that isn't a diaper, could it?
"You little..." comes through under your breath.
"Don't worry Bonnie, you're my sister. I'll help you learn how to use that tricky toilet, you just have to do what I say until I think you're ready." Sarah says giggling menacingly.
You really hate to give in, but you have no choice. You've been turned into a 16 year old girl in a soaking wet pair of boys underwear who doesn't know how to go to the bathroom or change by yourself.
"Fine Sarah, what do you want?" You ask grimly.
She grins and claps "I think the first thing that needs to be done is you getting some new toys with your own money."
You're ready to shout no, but then you remember you have no choice.
"Alright" but then you start to feel your bladder tightening, "uhg, not again."
Sarah walks over, "Oh right, you're not dressed properly, hehe." She grabs some clothing from your draws and turns into a new diaper.
"Now get down Bonnie, I think you've made enough mess for one night." You comply, and Sarah quickly changes you. With that you manage to get some of your clothes on (that Sarah girlifies) and go fulfill her demands.
Some minutes later, you come back. "Here, you spoiled..."
You feel warm and wet between your legs again. "Oh no, I'm wearing the stupid diaper! So what's wrong?"
Sarah giggles, "Why are you surprised, you're only wearing the diaper because you can't use the bathroom, hehe. You'll have to go every 30 minutes even if you are wearing one! But don't worry, even though you can't change yourself, I allowed you to be able to change your own dirty diapers, remember?"
You almost fall over in defeat, but then just deiced to