Research Findings: The goal of this study was to develop a construct of sibling cognitive sensitivity, which describes the extent to which children take their siblings’ knowledge and cognitive abilities into account when working toward a joint goal. In addition, the study compared 2 coding methodologies for measuring the construct: a thin slice approach (i.e., making intuitive, impressionistic judgments) and an interval coding approach (i.e., coding the presence of behaviors in 20-s snapshots). A sample of 385 sibling pairs (younger sibling M = 3.15 years, older sibling M = 5.57 years) was used for the present study. In Phase 1, independent raters used both methodologies to code videos of sibling interactions using a subset of sibling pairs (n = 50 dyads). Siblings interacted for 5 min on a challenging cooperation task, and the extent of cognitive sensitivity was coded for each child. Measures of validity and interrater agreement were acceptable for both methodologies, and thin slice coding reduced time demands. The thin slice measure was chosen as the preferred method. Phase 2 added 3 additional items to the thin slice measure and validated the measure using data from all 385 sibling pairs. Psychometric properties of the final thin slice measure were good. Practice or Policy: Research and practical implications are discussed.