I read this book I got at the 도서관, 라이브러리 during the summer and it was really good but I can't remember the name of it

I want to recommend it to my friend and what I can remember is that its about a
girl who sister was killed and she was given her diary 의해 her sisters boyfriend for her to read and she finds out that she got into a lot of bad stuff
if anyone knows of the book I'm talking about I would really appreciate i think the cover had blue and green on it