This doesn't sound right.

If a minor makes a report of abuse to a therapist, said therapist is a mandatory reporter, even if she feels that the minor is lying. That is, the therapist does NOT get to make the call as to whether or not it is the truth. I somehow don't think that keeping all of this hidden is the right thing to do.

I have no idea who you contact in a case like this, but you need assistance outside of this therapist. I think that if the therapist keeps this quiet, it is a breach of ethical code and she could lose her license. She is not handling things in the correct way, not legally and not according to whatever professional association she belongs to.