Just out of curiosity, why does your mom want to go with you? IDK about the legality of your T saying it's OK for your mom to come to the appointment.

Can you maybe see about splitting the appointment so that you can talk with your T before your mom comes in? Or at least contact T by phone or email and set some ground rules?

B/c I tend to be sarcastic, I'd tell your mom to get a T of her own and you'll be happy to join in on her appointments.