Withersmith

I cant remember exactly where I read this story, but the plot was basically a father left his daughter over at someone elses place for the summer. The persons place she was left at had two daughters, and one was younger than the MC and the other was older. Something lead the MC to be diapered, but, unknown to her, the rash cream they used actually made her more and more incontinent. Later in the story it was revealed the cream itself was developed by the older daughter of the house and she was some genius scientist developing other things as well. Another side thing going back to the rash cream is that the younger daughter knew about its effects and intentionally “sabotaged” the MC.