This study explores adverse childhood experiences and problematic psychosocial functioning for youths who exhibit harmful sexual behaviours (HSBs) against siblings in comparison to those that sexually harm non-siblings. In a sample of Australian male adolescents referred to a specialised sexual offending treatment service, greater prevalence of all adverse experiences investigated, except for caregiver incarceration, was found for youths who harmed siblings. Youths whose sexual behaviours harmed siblings were differentiated from other youths with HSBs in regard to characteristics of caregivers, including caregiver mental health issues, caregiver sexual victimisation experiences, and instability in caregivers during childhood. In addition, youths who exhibited HSBs against siblings experienced a greater total number of adversities in their lives and were characterised by poorer psychosocial functioning, such as social exclusion and internalising problems. Implications of these findings for clinical treatment is discussed. PRACTICE IMPACT STATEMENT: This research identifies developmental risk factors of youths who exhibit HSBs against siblings and examines if and how these differ from characteristics of youths who sexually harm non-siblings. Greater understanding of youths who sexually harm siblings allows treatment and prevention strategies to be tailored to their specific needs.