littlegriff..........I do not think that it us a big a deal as you might think...........a big deal for you and your wife, but for her she will take it well.
I agree with stoke.....at an earlier age would have been better, but no harm done. Personally I would wait until she was 16 yrs old.
I was told that my sister was my mother when I was sixteen years old, but it was only when I was in my mid fifties did I want to know about my father.
Psychologists and others will be at your disposal, but I wish you well.
I never needed such help and I never felt under privileged and she will always look upon you and your wife as her mother and father.