The article analyzes the relations between adult siblings, their specifics and diversity, as well as some factors affecting these relations. A brief review of modern foreign research in this area is also given in the article. We have presented the results of empirical research of sibling relationships on the national sample (102 adults who have at least one sibling) from the point of view of their closeness, trust, conflict/hostility and other parameters. We used two adapted questionnaires — Adult Sibling Relationship Questionnaire (C. Stocker and coll.) and Lifespan Sibling Relationship Scale Items (H.R. Riggio). The article highlights and describes four types of relationships: close and non-conflict (45% of the sample), less close and more conflict (31%); indifferent (19%), not close and conflict-competitive (5%). In addition we assessed the impact on the nature of the relationship between adult siblings of such factors as sex, age gap, the order of birth, the proximity of residence and marital status. Women are more attentive to their siblings and know them better; men are more likely to recognize that admire brothers more than sisters; dominance and opposition expressed more in same-sex pairs of siblings. In childhood the less age spacing the more active interaction between siblings and the higher trust between them is. In adulthood this factor is of no importance. As a rule, the younger know more about their siblings; they lend more emotional support to their siblings and count on getting the same from them. Siblings who live near to each other have more competition, they are more supportive of each other in practical terms and they interact more actively and positively. Adults who are married have closer and less conflict relationships with their siblings.