The aim of the present study was to test whether relations between temperament, sibling relationship quality, and problem behavior and social competence of children at school entry are in line with either of two competing models, dual risk and differential susceptibility. Our sample consisted of 977 Dutch mothers (Mean age 35.7 years) reporting about a target child at school entry. Regarding target children, mean age was 4.7 years, 48.1% were boys and 52.1% were older than their sibling. Mean age difference between siblings was 2.7 years. Using a cross-sectional design, mothers filled out online questionnaires concerning sibling relationship quality, temperament, problem behavior, and social competence of the target child. Path analysis was used to examine whether temperament moderated the link between sibling relationship quality and child psychosocial functioning. In line with differential susceptibility, results from moderation analyses indicate that among children low in effortful control (EC), sibling conflict was more strongly positively associated with internalizing and externalizing problems than among children high in EC, but sibling warmth also was more strongly positively related to social competence in children low in EC than in children high in EC. However, follow-up Region of Significance analyses shows that our findings are only consistent with weak differential susceptibility. No significant moderation effects were found for surgency or negative affect. Our findings suggest that low effortful control is a susceptibility marker concerning the link between sibling relationship quality and child functioning. Limitations, implications, and future directions are discussed. (PsycInfo Database Record (c) 2022 APA, all rights reserved).