While siblings can be close allies, they can also be significant competitors; greater levels of conflict were reported by sisters, those closer in age, those who have co-resided longer, & full-siblings compared to half-siblings

Good Friends, Better Enemies? The Effects of Sibling Sex, Co-Residence, and Relatedness on Sibling Conflict and Cooperation. Catherine A. Salmon & Jessica A. Hehman. Evolutionary Psychological Science, Jul 19 2021. https://rd.springer.com/article/10.1007/s40806-021-00292-y

Abstract: While siblings can be close allies, they can also be significant competitors. They are also family members that are typically with us for most of our lives. Research has raised questions about the factors shaping sibling relationships, and an adaptationist perspective would predict a role for a number of factors including sex, genetic relatedness, and childhood co-residence. Recent work has highlighted sex differences with regard to conflict and emotional closeness, greater conflict among full-siblings than half-siblings, and a role for co-residence in increasing sibling altruism. This study examines levels of both sibling conflict and sibling cooperation as a function of respondent sex, sex of sibling, birth interval (or absolute age difference), co-residence, and relatedness. Results indicate that sibling conflict and cooperation may not be shaped by the same set of factors. Sibling conflict was predicted by own sex, sex of sibling, birth interval, duration of co-residence, and the degree of relatedness. Greater levels of conflict were reported by sisters, those closer in age, those who have co-resided longer, and full-siblings compared to half-siblings. However, sibling prosocialness was only predicted by sex and relatedness with females and full siblings reporting greater levels of sibling prosocialness. More research investigating patterns of conflict and cooperation within families using more ecologically valid cues are necessary to determine whether the two are operating under the same mechanism, sensitive to the same cues, or are, indeed, operating under different mechanisms.