hangingon, is there any way you can have less contact with your mother and her toxic behavior for the time being? How often do you talk by phone? If daily, could you switch that to weekly? If you speak weekly, change to once every 3 weeks? Or something like that. You are an adult. You can choose the frequency of contact. When you have dealt with other upsetting issues and feel stronger, you can resume more contact, if desired.

I agree, definitely share the concerns about your mother with your T.

You could also set some very firm boundaries with your mother. For example, "because of your past lack of support and negativity, you are not allowed to mention my counseling to me. If you do, I will hang up the phone." If she persists, don't answer the phone when she calls. If she really wants contact, she will abide by your rules.

For your sisters and you, you could go to a family therapist. You could see the therapist alone sometimes and with your sisters at other times, and they could see him/her individually too when they wanted. Family therapists are trained in seeing different combinations of family members from the same group. Maybe your current therapist is a family therapist or can refer you to someone appropriate.

My main advice: distance yourself from your mother at this time. Good luck.