Family members of offenders with a mental illness experience serious emotional, social, financial, and psychological burdens. They have to deal with both a rollercoaster of emotions and a double stigma for which they need support. However, until now, few family interventions have been developed in forensic settings because they face particular challenges. Therefore, a family support group (FSG) has been developed to support family members in dealing with both the mental illness and the judicial order of their relative. The present article describes the treatment protocol of an FSG which is based on a multi‐family group format that integrates systemic and narrative approaches and is adapted to the forensic context. The organisation of the FSG is explained and clinical vignettes are used to illustrate the therapeutic process. Finally, some reflections about the FSG are discussed.