When I was around that age I watched all of the FMA series that was out at the time. I also watched the entirety or other anime such as Naruto and One Piece.
These anime did not affect my religious beliefs.
If you want, you could watch the anime with her, and if anything inappropriate appears, you could say that the episode that is on is bad and that she shouldn't watch it. You could also explain that there is only one God (I'm having this issue with my younger brother right now, because they are studying Greek Mythology at school -- he's only 8 and taking the teachings literally) and that the show is fictional, that Magic is against the Bible, etc. If she sees it as just good entertainment, it won't do much harm.
Forcing her to not watch the anime, in the name of Christianity, may be a worse decision. I've noticed that kids with the free will to choose what they want to do remain much more faithful than ones with over-protective parents.
There are some parts in that series, if I recall correctly, that are actually pro-Christian. The main character has a dual with the characters that are themed and named the "Seven Deadly Sins", and wins by calling upon the righteous style of combat.
(I hope that this post is not taken the wrong way. I do not mean to bash JasonCran or any other adult or teen that believes you should not let your soon-to-be step-daughter watch Anime. This knowlege comes from my personal experiance. To paraphrase: I went through a phase of watching anime, It did not change my religious beliefs. It was just good fun and I would make-believe that I was a ninja. Now, all of my friends also went through this phase. These friends are now the ones who most actively research and practice their religion, being what it may, at my school. As opposed to kids who were "sheltered" and are now rebelling agaisnt their parents via sex, drugs, and religion)
