I have to admit I've always imagined her half hispanic just because of how Stephenie describes her, but I don't remember that being mentioned in the book. Where did آپ read that? because it would be so cool if it was true (I'm mexican):D
Is in the book, she was born in Albuquerque, New Mexico. That's why I don't like the idea of Sophia بش playing Melanie. Anyway ... I can't say in wich page it is, but I can say that is in the third chapter of the book, when Wanda begins to tell what she knows about Melanie (to the Seeker).