What does "Joining" look like/mean when working with a family using "Structural Family Therapy"? How would "Joining" apply to the family of Kel, Hal, Sierra, Mat, and Kile?

In "Structural Family Therapy," developed by Minuchin, a therapist uses "joining" to understand the dynamics of the family and to disrupt its unhealthy functioning to create a healthier dynamic among family members. The therapist is a force for change within the family system. The underlying idea of Structural Family Therapy is to understand the subsystems in the family and how they affect individuals in the family.
In this case, there are many subsystems that the therapist could join. For example, he or she could join the subsystem comprised of the dad and his biological children (Mat, Kel, and Hal) and their missing biological mother. Using roleplay and other tools, the therapist could help them understand the role of their missing mother and the effect she continues to have on the family, even as an absence in the family. The therapist can also "join" the unit of Sierra and Kile and examine the role that Sierra's abandonment of Kile years ago plays in their relationship and in the life of the larger family. The therapist might join these subsystems before joining in the larger family and examining their functioning, including the role that Mat's economic pressure and depression plays in the family and the role that each of the siblings plays. Can you think of other subsystems in which the therapist could "join" the family?