I'm currently reading The Madman's Daughter (to be published February 2013) and it's got a familiar love triangle, the one we see in The Hunger Games and Twilight.
This triangle is presented when our protagonist (Bella, Katniss) is at a transformative stage of her life, 16 or 17 or 18, on the precipice between childhood and adulthood. She must choose between a young man she has known all her life and a young man she has just met. The one she has known all her life (Gale, Jacob) represents childhood, the familiar, safety. He's almost brotherly. The new guy (Peeta, Edward) means change, what our protagonist can become, adulthood. Edward and Jacob don't need to be fully fleshed characters because their role is to mirror the protagonist, to present an integral choice. Does our protagonist choose to go with the safe option, or does she choose new experiences? Will she allow herself to mature, or will she remain a child? (Hunger Games spoiler alert) The Hunger Games irrevocably changed Katniss; she can never go back to that girl hunting in the woods with Gale. She has to choose Peeta.
When I finish The Madman's Daughter we'll see if I'm right. Will Juliet Moreau choose brotherly Montgomery, or as I think will happen, the mysterious castaway Edward?