What is the name of this book?

[ekulr]

What is the name of the young adult book with a character named Brianna Knox, or Bree Knox. Then there is a set of twins who don't know they are twins and grew up without any knowledge of the outside world. And when they went outside the walls, a burn corpse was sent back to the rest of the villages. 18 year olds also disappeared on their birthdays. There are also rebels who fight against another group who control the people.