Exposure to family violence during childhood has been linked to long-term psychological problems such as depression, aggression, and posttraumatic stress. Many studies have explored direct maltreatment and witnessing interparental violence, but there is much less literature exploring the impact of witnessing sibling abuse. Given the importance of sibling relationships, sibling relationship quality may play a moderating role in the relationship between exposure to violence in the home and adult outcomes. We were particularly interested in sister-sister dyads and the potential for positive sibling dynamics to offer protective effect. Two hundred twenty-eight women college women (mean age = 20 yrs.) participated by completing the Sibling Relationship Questionnaire (SRQ), Conflict Tactics Scale 2-Sibling Version (CTS2), Traumatic Events Scale (TES), Trauma Symptoms Inventory-2 (TSI-2) and additional measures as a part of a larger study exploring the longer-term psychological outcomes of witnessing sibling abuse during childhood. We ran a series of hierarchical multiple regression analyses to examine the relationship between witnessing the abuse of a sister in childhood and each of the composite scales of the TSI-2 when other forms of trauma were statistically controlled. Witnessing a sister's abuse during childhood accounted for significant unique variance in the externalizing behaviors of adult women when other forms of trauma were statistically controlled. Externalizing symptoms include irritability, anger, and aggressive behaviors. Moreover, the effect was moderated by sibling relationship quality in that this association was strengthened by lower levels of childhood conflict with the target sister. Importantly, witnessing sibling abuse did not account for additional unique variance in internalizing, somatizing, or posttraumatic stress symptoms. In addition, lower conflict with the target sister was not protective, but exacerbated later externalizing symptoms. These findings highlight the lasting impact of witnessing childhood abuse and the important role of sibling dynamics, especially when considering young women with externalizing behavior.