Due to paternity uncertainty siblings cannot be certain that they share paternal genes. To solve this adaptive problem, natural selection could have favored the development of kinship recognition mechanisms based on cues that have been reliable indicators of genetic relatedness in evolutionary history. The aim of this study was to investigate the role of physical and psychological resemblance as phenotypic kinship cues in explaining altruism and conflict – behaviors that are typical for sibling relationships and that are expected to vary based on genetic relatedness. The study was conducted on a sample of 395 participants. Participants completed an online questionnaire that consisted of socio-demographic questions, items that measured perceived physical resemblance, perceived psychological resemblance, emotional closeness, everyday investment, conflicts and of six vignettes that measured willingness to help a sibling in a hypothetical situation. Physical and psychological resemblance were positively correlated with everyday and hypothetical investment, but after controlling for socio-demographic variables and emotional closeness they no longer had an independent contribution in explaining the variance of everyday and hypothetical investment. Women, those whose sibling is younger, those who communicate more often with their sibling and those who are emotionally closer to their sibling report having greater levels of everyday investment in their sibling. Emotional closeness was the only significant predictor of hypothetical investment. Physical resemblance was not correlated with conflicts while psychological resemblance was negatively correlated with conflicts. None of them had an independent contribution in hierarchical regression analysis. Individuals whose sibling is younger, who live closer to their sibling and who feel emotionally less close to their sibling report having greater levels of conflicts with that sibling.