In the Twilight series, why is it that Stephenie Meyer's vampires sparkle in the sun instead of being burned by it?
Meyer never gives an exact explanation for this variance from the traditional vampire other than the fact that she got the idea for the story from a dream. She dreamt of the meadow scene where two people were having an intense conversation about their doomed relationship. One was your average American girl (Bella), and the other was a boy who was "fantastically beautiful, sparkly, and a vampire" (Edward).
Meyers did not research traditional vampires when she began writing her book. Hers come from her imagination. As a matter of fact, there are very few similarities between her vampires and those like Nosferatu or vampire bats. Other than the fact that they are immortal, pale, and drink blood, her vampires are very different from "normal" vampires.
All of this is fiction. People have different ideas of vampires, some may say they feast off blood, others say they feast off emotions. In this case vampires cannot bear to be in the sunlight while in other cases they get burned. However, vampires can pull back together which is why they need to burn to avoid regeneration.