The Taking of Annie Thorne — C.J. Tudor

One night, Annie went missing. Disappeared from her own bed. There were searches, appeals. Everyone thought the worst. And then, miraculously, after forty-eight hours, she came back. But she couldn't, or wouldn't, say what had happened to her.
Something happened to my sister. I can't explain what. I just know that when she came back, she wasn't the same. She wasn't my Annie.
I didn't want to admit, even to myself, that sometimes I was scared to death of my own little sister.