My daughter and I read the series together, because I had heard that the books had some questionable content.
My dd is 12, and she is a very mature 12, and I had no problem with her reading all 4 books. Yes, the 4th book does have sexual content, it is not gratituous (sp) or explicit, and in that paticular book it is pivitol to the plot. Like the other poster mentioned, I appreciated that the characters were married before the sex scene. There is some teasing in the 4th book also about wether the 2 main characters are or are not having sex as well.
I assume at this age you have had the "talk" with her, and not just the basic details, but that sex is reserved for marriage, that you have sex because you LOVE someone, and that yoru sexuality is a gift, and the more people you give it to, the less special it is. I am comfortable enough in my daughters self-esteem, vision for her future, and self image that I allowed her to read the 4th book after I made sure I was comfortable with the level of sex in the book.
I admit, I have to laugh a little about the parents who mentioned how Bella set bad examples, of course she did - it's a book!! Personally, I do not worry that my daughter is going to read the book and decide she wants to have a relationship and sneak around behind my back.
I don't worry that she will watch Suite Life of Zack and Cody and decide to pull hairbrained schemes, or watch Phineas and Ferb and decide to make things in the backyard behind my back, or read Diary of a Wimpy Kid and want to tp someones house, or read Felicity an American Girl and decide to steal a horse, or read How to eat Fried worms and decide to eat a worm, or watch Hannah Montana and decide to manipulate people into getting what she wants.
In fact, I'd rather she read all those things in books and live vicariously through her books. I read all kinds of books, mystery, vampire novels, horror, sci-fi, drama, romance, suspense -- it doesn't incline me to cheat on my husband, try to find a far off world, murder someone, etc, etc, etc.
Now, of course, that all being said, I encourage you to read the 4th book for yourself to judge wether you think it is appropriate for your daughter based on what you know about her. You know her better than anyone else! And you may find that you really enjoy the series. I really enjoyed being able to read the series with my daughter adn have discussions with her about it. We both went to see the movie and rather enjoyed it, there are a few "killing" scenes in it, none of which are graphic, but that will be up to you on wether or not you feel that is appropriate for your daughter. We are movie buffs in this house, all my kids (ages 12, 9, 6, and 4) have watched movies like The Lord of the Rings, etc and not been bothered by the graphic killing scenes because we have always shown them the "makign of" so they see it is not real and they love to comment on the special effects - so none of my children would perceive the scenes in Twilight as scary b/c they know it's just a movie - but you will have to judge wether or not your daughter has that kind of comfort level.
I thought the series was beautiful. I thought Bella and Edward had an amazing giving love for each other, I thought the deep rooted friendships were wonderful, I loved the closeness of the Cullen Famiy and loved how all the characters were ready to make a sacrifice for each other. Sure the characters run away and rebel, but there are a lot of positives in the book as well, and there is a very strong sense of family values and family bonds. You also read a lot about some of the characters struggles with humanity and it resonates very well with even the most basic of us.