This study examined the similarities and differences in the adolescent monozygotic (MZ) twin and same-sex dyzygotic (DZss) twin sibling relationship. Specific constructs investigated were: companionship, empathy, directiveness/teaching, avoidance, rivalry, and aggression. Participants included 192 same-sex twin pairs, age 9 - 18, and their biological parents. The data is part of a national study from the Non-Shared Environment in Adolescent Development Project. The nationally representative data set provided a measure of the Sibling Inventory of Behavior - Expanded Version (Anderson & Rice, 1999), whereby each participant individually responded to the paper-pencil questionnaire. Twinship status differences, gender differences, reporter differences and reporter by twinship interactions were examined. The results found significant differences between MZ twins and DZss twins on the constructs of empathy, companionship, avoidance, rivalry, and aggression. MZ twins demonstrated and reported higher levels of empathy and companionship and lower levels of avoidance, rivalry, and aggression, when compared to DZss twins. There was no interaction effect of twinship by gender, except on directiveness/teaching; however, significant differences were found between male MZ/DZss twins and female MZ/DZss twins on the constructs of empathy, companionship, directiveness/teaching, and avoidance. Female MZ/DZ twins reported and demonstrated higher levels of empathy, companionship, and directiveness/teaching, and lower levels of avoidance. Overall, there were no reporter differences between twins on any of the six constructs. There were significant differences between parental perceptions and twin perceptions on empathy, companionship, directiveness/teaching, rivalry and aggression. A significant difference between mothers and fathers was found on the constructs of empathy and avoidance. On the construct of avoidance, mothers and fathers differed from each other, however, fathers did not differ from the twins. There was no twinship by reporter interaction effect, except on directiveness/teaching. Overall, the results suggest that MZ twins report and are perceived as having a more positive relationship than DZss twins. The results also support the need for multireporter research in order to obtain a truer picture of the family relationship. The gender findings further extend the understanding of the role of gender in the sibling relationship. Lastly, due to the paucity of twin research, the results aid in the development of a framework for understanding the adolescent twin relationship.