Hello there,
This is a difficult situation but not one you should be afraid of. I think it's important to keep things in the open and talk about them. Your daughter needs to be prepared for such a meeting and I think it would be helpful to explain the situation to her and to see how she reacts and how she feels.
I am a great believer in letting children take part in the decision making process. Otherwise children feel left out and a few years from now your child might feel anger towards the whole affair. The bottom line is that you're her mother with everything that stands behind those words. This is a situation that comes up sooner or later in every family that has an adopted child, even if the biological mother isn't around, for there is a constant search of identity, in adopted children's lives.
I hope this meeting goes OK and that it will bring good things with it.
All the best to you and your family.