We don't know who is talking, but the first line is, "In my first memory, I am three years old and I am trying to kill my sister" (Prologue.1). Yee-haw.
The nameless narrator also says "I didn't seem to exist, except in relation to her" (Prologue.2). Elle Fanning, is that you?
Finally, she says, "I did not kill my sister. She did it all on her own" (Prologue.3). This plot is as thick as London smog already, so let's get started.