I agree with Perna. It's your therapy anyway, you should be able to choose the topic you discuss but maybe your therapist doesn't quite understand how important it is to you to talk about him. A good therapist will at least try and figure out why you want to talk about him, whether or not she thinks it is the most important topic. Be deliberate about bringing it up and if after that she still guides the conversation towards your mom, ask her why. She might have a good reason but if not, then maybe find someone else.

Good luck