I think there's about as much chance as Twilight leading young people on to healthy relationships as there was of Elvis Presley turning teenage girls into whoreswhoreswhores back in the day. People grow up and mature and can recognize the unpleasant messages any media conveys. If Twilight promotes unhealthy relationship messages than we need to throw out Wuthering Heights too. And Othello. Or any other book in the Western canon that doesn't portray perfect relationships.
But Twilight is far more subtle. Bella’s and Edward’s relationship is treated as a normal and acceptable one. In other books tumultuous and unhealthy relationships have some negative result. In Othello, for instance, everyone ends up daed.
And can you be sure you’re speaking for all 12 and 13 year old girls who’re still reading these books?