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Based on a clinical case, this paper sheds light on what underlies adolescent sexual transgressions involving siblings. Specific elements—pertaining equally to behavior, discourse, and the unconscious—are at work in adolescents who commit sexual transgressions. Their egos seem to be split along the lines of depression and perversion. This paper also discusses the strict setting in which evaluation and treatment occur. Various forms of dialogue are needed to address the numerous questions pertaining the passage to the act. The complementarity between behavioral, systemic, and psychodynamic approaches affords a broad scope for understanding. Throughout the pluridisciplinary clinical approach, our concern is to maintain an ethical awareness of the adolescent’s strong developmental mobility and of his family.