YA book about a girl with a weak heart that carries around an AED/Defibrillator at school. Spoiler ahead.

[Mika]

It was about a teenage girl who was very sickly and anything could set her heart to fail. She had a dad and stepmom who was a nurse at her school. She carried around an AED/Defibrillator in a bag at school in case she had any heart problems. We find out that she had either a twin brother or maybe it was just a younger brother (I can't remember) who died when they were kids because their stepmom was poisoning both of them and was still poisoing her which is why her heart was so weak.

I first read this book on Wattpad. I don't remember when but it couldve been between 2014-2017. The author didn't post all the chapters because her actual book was in stores so I ended up reading it after I bought it. It was in the YA section at barnes and nobles.
The think the cover of the story had a heart on it.

[Becca]

Sounds like Broken by C.J. Lyons. Some of the reviews reveal the twist ending e.g. this one - https://www.goodreads.com/review/show...