It is going to depend upon what you do and do not want your daughter to see, and more importantly, to experience. I have not seen the film myself, but if you have doubts about whether she should see it, I would suggest either seeing the movie yourself, before you make that decision. If you just want to know what is in the movie that might give you pause, go online and look through "Twilight: Breaking Dawn" spoilers--that will give you a synopsis of the plot and everything which might concern you. Then, you can make an informed decision. But if you're that worried about it, don't let her go.
It's PG-13. Your the Mom. If you want her to see it go with her. If you don't tell her NO. If that's not the end of the conversation then make it the end of the conversation. Your the MOM. If she can't wait until her birthday to see it then it's more about peer pressure than about her wanting to see a movie. YOUR THE MOM. Show her how little peer pressure should affect her decision making.
If you let her see other pg-13 movies then yeah go ahead. My parents always let me see pg-13 movies when i was 12. Apparently bella's labor scene is gonna be graphic tho... I'm not sure what whoever told me that meant... You might wanna think about that...
Is she a mature 12 year old? If so, then go with her. Discuss the movie afterwards and ask her what she thinks about the different scenes in the movie. You two can exchange opinions and dialogue. She needs parental guidance and that's what you need to provide. If there are scenes you disagree with, tell her and why. This way, she will not rebel because you are not a dictator but a loving parent who guides her. When she grows up and have her own kids, she will do as you did.