hmm, my earlier response to this seems to have disappeared...
I would suggest talking to your t alone at the start of session, so you two can get on the same page about how things go, and maybe get some coaching on how to talk to mom (or if/when you would like T to step in and talk).
I've only ever had people come to my sessions after being invited, or after asking me directly. I'm not sure if you feel comfortable with your mom being there, or if she "has" to be there (if you are a minor, or there is another reason she has to be there even if you don't want her in the room), or what, but you are still the one in control of the session. You can talk to T before hand about not only how to talk to mom, and about what, but also what your limits are and what would be reasons to ask mom to leave the session.
A long time ago, I took an s.o. to session with me, and when I started to dissociate pretty badly because of the conversation, T asked my s.o. to step outside for a bit. My s.o. was pissed beyond belief, but did leave the room and my T was able to help me ground. We talked a bit about what happened and brought my s.o. back in to continue, but with a different focus.
Other times, I brought my mom and/or my wife in with me. T and I always took some time at the beginning of session to check in and "set ground rules" for the session. That was also the time I asked t for help in explaining some things. Only once did a family member accompany me when I didn;t really want them there. The T and I met before hand, and I was able to tell them I didn't want that family member in session with me. I told her I was worried about backlash from the family member if I out-right told him he was not welcome. T and I never did invite him in, and at the end of session, T "lied" and apologized to my family member for "losing track of time" & not inviting him in. He was then mad at T, not me, and didn't attempt to come to more session after that "waste of time"...