I remember reading this great trilogy series about 10-15 years ago. It's about a family living in a dystopian walled city where people are sorted and classed by their intelligence.
The protagonists are twins, a boy and girl. They defy the system and are exiled from the city, along with another boy. They journey across a desert and have many adventures etc. This is only in the first book.
I remember the cover being a big 'S' shaped symbol. It had something to do with a wind chime that was central to the plot.
Edit: There is already another question with the same answer here. I did do a search on stackoverflow before I posted this question of course, but I feel I would never have know it's the same question just looking at the title. It's up to the community to close this question as a duplicate.