I just want to say don't make your child feel like she has to choose or that she isn't allowed to call another woman mommy. Its difficult, yes, and it hurts but being that child who had two moms is even more difficult. There is pressure on the child that you may not know about. Just be happy that your child has a big enough heart and is sharing her love with her whole family, new additions and all.
I was in the same spot as your child is. My father remarried (to a much nicer woman than my own mother) and I immediately wanted to call her mom. My biological mother treated me horrible for it (along with her already being abusive mentally and physically) so Please do not put any added pressure on your child to force her to call her step mom by any name-let her choose.
You've got to understand its going to be all right.
Think about it, there will be a day when she says to the step mom "You're not my REAL mom" even though this woman is helping raise your child along with you and the father-not asking for anything in return. Its going to hurt her to have to hear that one day.
So just keep in mind, I'm sure the step mom feels very loved and accepted by hearing your daughter call her what she is calling her but she [your daughter] can't even fathom what is really going on in the grown up world when it comes to this issue.
Just let it be and everything will turn out okay.