Okay, I'm a little bit confused.... have you and your husband had any relationship with this little girl at all? Isn't her mom the person that she has had the longest running, deepest relationship with up to this point? If so, I think her mom should be the one to share this information with her.
Before she talks to her daughter, I would suggest the 3 of you get together and go over how information should be delivered. Something along the lines of, "Family takes many forms," and "A father is someone who loves you, cares for you and raises you, but a father can also be someone who you share genes with," or something like that.
The 3 of you may also want to consult with a child psychologist about this issue to get some direction from her/him about how to approach this subject with your stepdaughter, the steps you can take before sharing the information that will put her a better frame of mind to be able to receive what you want to tell her, and whatever follow up messures should be taken to insure that she has the emotional support that she is probably going to need to take this all in.
How she reacts to this information depends on a lot of things. She's old enough now to understand most of what this is all about but she may need some time to process all of this information. If she is an easy going kid, she maybe like, "Oh, okay." If she is more tempermental, then she may feel some sense of anger or resentment. Or she could have suspected all along that her stepdad was not really her bio dad all along.
I hope this helps. I think consulting with a child psychologist would be a really good idea before you share all of this info with her, especially since her mom is having issues with her already. This is just what I think would be best.
I hope you and your family have a wonderful Thanksgiving weekend.