This is an excellent question, thank you for asking.
I can completely empathize with your reaction and frustration. The difficult part is that the therapist, unless there are very specific indications that the client is going to hurt themselves severely, or someone else, cannot breach the confidentiality of the therapy sessions. Confidentiality is the basis for much of how and why therapy can work.
Also, therapist training may be very different so a therapist proficient in individual therapy may not have any skills in family, couples or group therapy and both ethical and legal sanctions prevent therapists from operating outside of their specialty. You wouldn’t want a dermatologist who believes you have skin cancer to perform surgery on you if he or she hadn’t had the training. So someone who is working one-on-one with an individual may not be familiar with family therapy to the point where an intervention or meeting with family members independently would be helpful.
The other hurdle is the fact that if the therapist pushes for other people to come in (assuming they have the skills for this), this can backfire. If the client goes along with it only because the therapist suggests it and the meeting doesn’t go well (and sometimes even if it does), it can undermine the trust in the therapeutic relationship.
I am assuming that the therapy is all outpatient, which means that the therapist has to follow very specific legal, ethical and insurance guidelines about what he or she can and can’t do. Infringement of these could result in the therapist losing his or her license. This is how important it is.
But I do think there are ways around it. It doesn’t have to be the same therapist that does the meeting. Family members can suggest to your sister that they would like to have a family meeting as a way of helping, and hire a separate family therapist for this. This would leave the primary, individual therapist-client relationship intact, while allowing some information to be shared. Again with your sister’s permission the family therapist could then talk to her individual therapist.
I know this will sound like a lot of mumbo-jumbo, and in many ways it is very restrictive and difficult to navigate the process of therapy. But these rules and regulations have come about in the interest of client well-being and safety.