A 16-year-old boy is referred for oppositional behavior and declining grades. During a family session, the therapist observes that the mother frequently answers questions directed at her son, the father sits apart and checks his phone, and the patient's 12-year-old sister intervenes during parental disagreements to calm both parents. The therapist hypothesizes diffuse boundaries between mother and son, rigid boundaries around the father, and triangulation of the daughter into the parental subsystem. Which family therapy model guides this formulation?