When our son was 3 years old he also went through that phase (and our son at 4 as well). What was very helpful for our boys was to hear it FROM the mommy in question that they are going to stay where they are. Our boys too had been prepared to return to birthparents and of course, suddenly (to kids anyway) got us (new mom and new dad). What we did, and you may consider, is having birthmom address that to your daughter directly. Either in person (probably best) or over the phone or in a letter form (also good so that you can go back and read it over and over again).
I would talk with birthmom and explain that your daughter may need permission from birthmom to fully relax in her new family. I think that most birthmoms would be willing to do that for the sake of their child. Even our boys birthmom (and foster mom) wrote a letter to them saying how sad she was that she couldnt be their mommy anymore but how happy she was that they have a mom who loves them so much.
There are also some story books that address that subject ... and they are great
