Dead Girls Don't Write Letters — Gail Giles

Things had been getting a little better until I got a letter from my dead sister. That more or less ruined my day.
When Sunny's older sister, Jazz, ran away to New York, Sunny was secretly relieved. Everyone loved Jazz, talked about Jazz, wished they were friends with Jazz. Jazz was perfect and Sunny was...well, not Jazz.
Then Jazz's apartment building burns to the ground and she is presumed dead. Sunny's family, already broken by divorce, unravels. Dad's drinking skyrockets, and Mom's depression hits an all-time nonfunctioning low. Sunny is left to cope.
Then they get a letter from Jazz saying she is coming home. But how? Jazz is dead, right?