I saw the recent Pirates of the Caribbean movie a few weeks ago. Loved it, but this post is not a review of On Stranger Tides. Rather, after seeing the Mermaids as presented in that movie, I had a revelation: The Sparkle Vampires in the Twilight movies are not really vampires they are mermaids (and mer-men).
Logic: The Twilight Vampire sparkle in light (and sunlight). This sparkle is a holdover from their ancestral roots when more of their bodies were covered by scales. Real Vampires are typically harmed by sunlight; only few Vampires should be immune or resistant, not all of them. Drinking blood? A substitution for actually feeding on the flesh of creatures (humans, etc.) to try and hide their nature more. Naturally the actual origin of their species was lost ages ago as they became what they are. Fangs? Check. Supernatural abilities? Check. Perfect match.