The adult survivors of sibling incest and assault (n=49 women; n=24 men) were recruited from clients of the San Diego Department of Social Services. Twenty-nine had experienced sibling incest; 26 were survivors of sibling assault; and 18 men and women reported a combination of sibling incest and assault. The data included transcribed comments and information obtained from the Adult Sibling Abuse Survivor Questionnaire. The interview addressed family characteristics that contributed to the development of abusive sibling relationships in childhood, how various family configurations affected the relationship between siblings in abusive dyads, and how sibling incest and assault in childhood affected the adult survivor's ability to form healthy relationships. After examining the nature and frequency of sibling incest and sibling assault, family configurations in cases of sibling abuse are examined. The configurations identified are peripheral parent families, in which one parent maintains a role exterior to the other; pseudo-consensual sibling incest families, in which siblings turn to one another for nurturance in the face of parental abuse or neglect; pseudo-parent sibling families, in which a sibling assumes a parental role in the absence of reliable parents; and stepsibling abuse. Regarding assessment and treatment, the authors conclude that the harmful consequences of sibling incest are as severe as father-daughter incest. Thus, the traumatic effects of sibling abuse warrant assessment and treatment in order to mitigate the long-term mental health consequences. 1 table, appended interview instrument, and 51 references