The book is written on two levels and one of the conclusions is that an imposter did come to the house, she did assume identities and she will assume Sunny's identity next, which puts Sunny in real danger.
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. The author 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 blink of her reality break and still writing letters to herself.