just want to say and good luck.
i was 7 i think when my mum told me my dad, wasnt my "real" dad, they were never together, and she had been with my step dad since i was about 18 months. i remember she sat me down with my baby book, and showed me a photo of a man in a army uniform standing between grandma & gramps, she explained how the "step dad" thing worked and how the guy in the pic was my "biological father" but also made it absolutle clear that it did not change a thing, i had asked my step dad when i was 3 if i could call him dad, he was wraped, and if i wanted to continue to do so, it was my decission. he wasnt there for "the talk" but gave me a hug and asked how i felt after i "knew"
good luck, and your right, she dose need to know! i remeber i felt a little wierd for a few days or so, but after that, life went back to normal and i never realy thought about it untill i was a tennager and mum started with the "ill send you to live with your father" i met him for the first time when i was 14, but still refer to my step dad as dad, even tho he and mum have been split for arround 8 yrs now.