If you mean advanced as in she can read long, old-fashioned books, I would recommend some of the old classics - Perfume by Patrick Suskind is fantastic, so is Wuthering Heights by Emily Brontë and anything by Jane Austen.
Clan of the Cave Bear is good, but a little explicit, so if you're okay with your 14-year-old sister reading sexually explicit things, then that is a good one.
I also recommend anything by Sara Douglass, more specifically the Troy Game series. They're really thick, and they're historical fiction/myth if that helps. Some of my favourites! (They're a little explicit too, to carry on my earlier warning).
Hope this helps!