One thing I have learned is that some of the books I loved as a kid don't work so well across generations. So I wouldn't recommend My favorites to your 12 yr old niece.

My 16 yr old daughter loved Hunger Games and she also loved the Uglies series (Scott Westerfield). She really likes Orson Scott Card, but I think she might have been a little bit older before she first read Ender's Game. It might be tough going for some 12 yr olds, but maybe not your niece, since she's a voracious reader.

In the same vein as Hunger Games, try The Giver and it's sequels, by Lois Lowry. She loved all of those too.