Now, my alarm goes nuclear.

If the memory reflects the real event, then you were sexually abused as a child. I would absolutely talk about it with your T, AND I would withdraw my consent for her to talk to my mother. You are an adult now and your therapy should be your sacred, well protected personal space. In your situation especially no one should be allowed there. Again, if the memory reflects the real abuse, then it means you have never been allowed to have your own protected personal space (and it certainly looks like it from what you've described in other threads), so it is time to claim one and to defend it.

I'd tell the T about the memory and about your father's current behavior and I would pay great attention to her reaction. A therapist with any basic level of ethics and wisdom in this situation would stop all communications with your mother and would work with you on helping you separate from the harmful family dynamics you are in right now. That'd be my litmus test for whether I should continue to see the T. Also, a T in this situation should assess if your father has access to children, and, if he does, the T should report his behavior to the appropriate authorities immediately. This is another test to see if your T is appropriate for you to continue to work with.

I am not telling you what to do. You obviously can and will make your own choice, but I am not going to pretend that I don't have a strong opinion about cases like yours. I certainly do and I never hide it. In cases like that, a strong opinion indicates a moral stand. There is no "gray area" for me here. This, to me, is as black and white as it can get.