I probably would have denied it to. I am not sure that's the point. In the asking, or talking around it as
@Friday mentioned, creates an atmosphere where the daughter can bring it up later. The other thing to think about this is that the Mom knows. That the information has been shared in the extended family through the step-sister. Acknowledging that in some way, helps take away the sense of secrecy. If the daughter finds out that the mom probably knows and hasn't said anything? That can make it harder to bring up.
Of course, getting the input from the therapist is also very important.