[TOMT][novel] A story about two sisters who are rapidly de-aging.

Okay, so in seventh grade (around 2004/2005) my homeroom teacher read us this book.

It was about two sisters, I can't remember their names but one was older than the other by a few years. They were both really elderly, like in their 80s or 90s, and they come across this doctor or someone who has a "de-aging medicine" or something. So, they take it, and they begin to get younger, until they're close to being toddlers/babies and unable to care for themselves.

Basically the book is about them trying to figure out how to reverse the process and start aging normally again, I think.

And that's about all I remember. Does anyone know this book? Thanks!