I picked up "Twilight" because my roommates told me that I'd love it and I wouldn't be able to put it down, not because I was expecting the next great American novel or anything of the sort. While I don't know if "Twilight" has any literary value (in whatever sense of the phrase you mean, that is), I do know that I've never heard anyone claiming that it does. So what if high school students of the future never end up analyzing examples of conflict in the books and underlining metaphors describing Edward's appearance as akin to David? And for that matter, "Harry Potter" isn't scheduled to reach the classroom any time soon, either. It's still highly entertaining when I want to lose myself in a fictional story.
While Edward has some appealing characteristics, including his looks and his propensity to look out for Bella, I'm not deluding myself into leaving my window unlocked hoping my knight in sparkling skin will climb in and watch me mumble his name in my dreams.
Maybe if you're so concerned about your sister and her friends, you should just remind her that all who drive Volvos do not sparkle and then let her and the rest of the world enjoy an entertaining fictional series about a charming vampire and the girl who's in love with him.