Within family sibling clustering of internalizing problems is examined during the early childhood period. Sibling clustering, the ongoing sibling similarity in internalizing problems, may be a result of heritability of internalizing problems, as well as shared environmental effects. Clustering may also result from the time-varying influence of sibling socialization, where 1 sibling is teaching or modeling internalizing problems to the other sibling. We compared a traditional cross-lagged panel model with a recently developed multilevel statistical model that differentiates the 2 mechanisms. Sibling clustering was operationalized as the family level, time-invariant variance in internalizing problems, and differentiated from sibling socialization, the cross-lagged (time-varying) association between earlier child behavior and later sibling behavior. A 3-wave longitudinal study tracked 916 children (age M = 3.46, SD = 2.23) in 397 families using a 2-parent composite score of internalizing problems. Results suggest the importance of accounting for sibling clustering in the context of a panel study, as its inclusion in the model eliminated the identified time-varying, sibling socialization effects. (PsycINFO Database Record (c) 2019 APA, all rights reserved).