The present study is the first to examine patterns in sibling relationship quality and the associations of these patterns with internalizing and externalizing problem behavior, as well as self-perceived competence, in middle childhood. Self-report questionnaires (e.g., Sibling Relationship Questionnaire, Self-Perception Profile for Children, Youth Self Report) were administered among 1,670 Dutch children (Mage = 11.40 years, SD = .83) attending 51 different Dutch schools. Three sibling relationship clusters were found: a conflictual cluster (low on warmth, high on conflict), an affect-intense cluster (above average on warmth and conflict), and a harmonious cluster (high on warmth, low on conflict). Sister pairs were underrepresented in the conflictual cluster and overrepresented in the harmonious cluster. Children with conflictual sibling relationships reported significantly more internalizing and externalizing problems, and lower academic and social competence and global self-worth, than children with harmonious sibling relationships. Children with affect-intense sibling relationships reported less aggression and better social competence than children with conflictual sibling relationships. Our findings indicate that it is fruitful to combine indices of sibling warmth and conflict to examine sibling relationship types. Relationship types differed significantly concerning internalizing and externalizing problems, but also concerning self-perceived competence. These findings extend our knowledge about sibling relationship types and their impact on different aspects of child adjustment. Whereas harmonious sibling relationships are the most beneficial for adjustment, sibling conflict mainly has a negative effect on adjustment in combination with lack of sibling warmth. Implications and future directions are discussed.