This text deals with a case study of an eleven years old boy who have witnessed the sexual abuse of his ten years old sister by their father. The theoretical referential is the Systemic Theory, in its intersubjectivity, contextualization and complexity scope. After having witnessed the abuse, the boy started to have nervous problems, presenting trembling legs. It was utilized, as instruments, the observation of the boy while in activity at a social service institution and an interview with his mother. The results focus on the suffering of the boy in his loyalty conflict: in his feeling and ties with his father he wants him back; in his feeling and ties with his mother, he understands that his father endangered his sister. How to solve this impasse? To love his mother and to hate his father or to be just like his father, losing his family?