The book is written on two levels and one of the conclusions is that an impostor did come to the house, she did assume identities and she will assume Sunny's identity next, which puts Sunny in real danger.
However, the other conclusion is that Sunny is an unreliable narrator. I wanted to set that up with her reaction to getting a letter from her dead sister. She isn't upset, shocked, surprised--any of the normal reactions one would expect--it "more or less ruins her day." This statement should let us know that something is wrong with Sunny--at least enough to be suspicious of her emotional reactions. Then I make a point of showing the reader that Sunny is devious--she can forge her mother's handwriting and does to write notes to the school--and she does Da Vinci mirror writing to keep things secret.
And how much of what Sunny says about herself makes sense? If Jazz was so scheming and evil--how was she so popular? Why was Sunny the ONLY person to ever see this side of Jazz? Why does Sunny have not a single friend? Why is a 14 year old left to take care of a mother, clearly so incapacitated, especially when Sunny can't even drive? And with a father, who, according to Sunny spends much too much time drunk and sometimes in jail for drunkenness? In this day and age if the school knew of this situation, as she says it does, wouldn't CPS become involved?
And the thing I thought gave it away--why would anyone let a stranger, who is obviously lying about who she is--into their home? That should have put the reader on notice that Sunny couldn't be trusted with this story. And another big give away. If Jazz was so self centered--why is her journal all about Sunny? And it mentions that Sunny would find a way to get away from her dysfunctional family.
The second level or second story is that Sunny has had a break from reality. She wrote the letter. None of this happened except in her head. I kind of left it a little open at the end--Sunny could have gotten psychiatric help as it says in the book that she does--recovers enough to go away to school--and then we know she has another break--she writes another letter to herself. Or--she's not in school at all, she's in a psychiatric hospital and still completely in the throes of her reality break and still writing letters to herself and now upping the ante of having the writer trying to take over Sunny's life rather than Jazz's.
In any case, I think the title of the book answer all the questions: Dead Girls DON'T Write Letters and it kind of shakes hands with the last line of the book when Sunny says, "What have I done?"
