I think you need to take your time, see how things go, I know you say you dont feel you can talk to your husband about it right now, but you must tell him how it is making you feel, this will only make things worse.
See how the DNA turns out, and if its is your husband's child he will then need to talk this through with the said mother and also involve you in this, this is as much your life as it is your DH & DSD.
I understand this is a shock for you and I am trying to put myself in your shoes and I would to be very shocked and upset with the whole situation.
However should the said mother allow your DH to have a relationship with this child and your DH wants this too, please dont stand in his way it's so important that the child has a relationship with a father.
Ladies this is a support group, construtive comments are advisable, and not to attack an individual. Gemz23 has come here for our support, you can do this without being abusive.
Gemz23 please keep us posted, on how things are going.