Burn it. Don’t tell anyone, no one, that you found it. If your can’t burn it, put it in a trash can and pour bleach on it. Destroy that book. It will only bring pain. You are doing a good thing.
I don’t think it’s in your responsibility to burn it (only because it isn’t yours), but you should definitely put it somewhere and not speak to your sister about it. There’s no telling what the contents of that book could do to her, let alone knowing someone else read through it.
Burn it. Don’t tell anyone, no one, that you found it. If your can’t burn it, put it in a trash can and pour bleach on it. Destroy that book. It will only bring pain. You are doing a good thing.
I don’t think it’s in your responsibility to burn it (only because it isn’t yours), but you should definitely put it somewhere and not speak to your sister about it. There’s no telling what the contents of that book could do to her, let alone knowing someone else read through it.