I am a father in a preachy-religious family. Only my middle child, a daughter, knows that I am an Atheist. She completely accepts me and has admitted to being a skeptic, but still a Christian.
I want to be an honest person with my wife and oldest daughter, but one upon a time I left Christianity to become a Jehovah's Witness. The fight my wife and I had lasted for several weeks and left me so wounded that I don't want to repeat it again. It was a horrible experience.
I feel badly that I have to fool her, but I also feel that a peaceful household is a good thing, even if I have to lie to get it.
I also fear that she may leave me if I admit the truth.
I am running a blog page that I hope she finds that explains why I am Atheist and dissolves some Christian mysteries using logic.
You can try to gradually showing her through actions rather than words (such as secular volunteer work) so that she can understand your side first hand without a direct confrontation. You could also tell her about your blog and have her read it by herself and hope that she will be understanding enough to see the toll that this deception is taking on you. I don't really know what type of person that your wife is or how exactly how irrational she might get to such news. You could also try and start with your older daughter and try to gauge her reaction and then she might be able to help you with telling your wife as well. I wish you good fortune in which ever path you decide to take.