split POV YA novel about two orphaned sisters until one mysteriously dies

[Julia]

I'm looking for a book about two orphaned teenage girls who are living on their own and going to high school. One day the sisters decide to throw a party and the youngest sister is shot and killed. Each chapter is from the perspective of a different person while they try to solve her murder. It starts at the party and then carries on through graduation. It's in the present day, not fantasy or anything, just normal kids. The two sisters are very wealthy. Read sometime between 2011-2013.