Purpose <p>Research has shown that sibling sexual harm (SSH) can result in psychological difficulties during childhood and adulthood. Yet the literature remains mixed regarding whether these outcomes may be different from child sexual abuse (CSA) perpetrated by other family members or those outside of the family unit. This study aimed to examine mental health functioning of adult survivors who experienced SSH as compared to non-sibling intrafamilial and extrafamilial sexual abuse.</p> Methods <p>Participants include 532 adult survivors of CSA who reside in the United States. Participants completed an anonymous, online self-report survey.</p> Results <p>Overall, we found that individuals who experienced SSH by a biological sibling experienced mental health issues including posttraumatic stress, depression, suicidal ideation, guilt, and shame. These symptoms were largely experienced at similar levels to those who experienced non-sibling intrafamilial and extrafamilial sexual abuse. Those who experience SSH reported greater distress in comparison to those who experienced extrafamilial sexual abuse. When combining SSH and other forms of intrafamilial abuse, those who experienced intrafamilial CSA had lower rates of guilt-related Hindsight Bias/Responsibility ratings compared to those who experienced extrafamilial CSA. However, there were no other significant differences in mental health issues reported by adults who experienced intrafamilial versus extrafamilial CSA.</p> Conclusions <p>The findings have important implications for both the detection and prevention of SSH.</p>