SPOILER WARNING!
Sarah Corcoran was the real target.
Because what the hell was she wearing to the library, to be mistaken for the Homecoming Queen?
Seriously, though, the only real evidence that Sarah was killed in Sadie's place is Peter Neilson's word, and he's a highly manipulative, malignant narcissist. It seems perfectly reasonable that he killed Sarah knowing the idea that she was mistaken for her sister would ruin Sadie's life... which is exactly what happened. He told Ellery that Sadie was the real target for the same reason: it amused him to mess with her in her final moments.
