It's troubling to see you say "least invested in" with regard to a 13 year old child. Whether a child expresses it in a way that is understandable to adults or not, a child still needs a mother and father.

Your step-daughter might say anything to a therapist. She should be able to say whatever she wants to a therapist. The therapist will figure it out. That is likely why she called your husband in. And what the therapist told your husband may not have come out the same way when he told you. We all hear what our individual brain's have prepared us to hear.

If this little girl fell down and skinned her knee, she'd likely cry. What she is doing is another form of crying.

It is upsetting, but your step-daughter needs love and support and it might be most helpful if you and your husband can go with the girl's mother to the therapist to find out the best way to support this little girl as she comes to terms with her parent's divorce and her father's remarriage. If that is not possible, you and your husband could seek out a therapist for yourselves for the same purpose.