I saw the film first before reading the series, so obviously I knew it was a lie because Hodge said so (though some film viewers seemed to have missed that line). Even if I was reading to start off, I think I might have suspected. It was obvious that they were still being hinted as the end-game pairing, since it was clear they both still loved each other and scenes kept appearing which made them kiss. And the only way to make that pairing acceptable would be to reveal that they're not really related, especially in a YA book.

I found out when I first looked up the book to see what it was about.