Re: DD has a "secret mommy"
We have a VERY similar situation with DSS. He calls his mother's husband "daddy" and my husband, his BIODAD "MY FATHER".
We have been over and over how this isn't his 'dad' and he shouldn't cal him that.
Needless to say he still calls him daddy, and prbly always will. All you can do is be supportive of her while she's little, as she gets bigger she'll figure out he's a useless idiot. And hate him for it. Just make sure you never say bad things about him to her, because then you'll also be the bad guy.
She knows you love her, she'll work it out. Its hard to watch your kids struggle emotionally, but its part of growth.