The current study contrasted two different hypotheses about the relationship between sibling personality and sibling relationship quality: absolute value and dyadic similarity. The absolute value hypothesis suggests that the level of one sibling's personality will predict sibling relationship quality. The dyadic similarity hypothesis argues that the similarity between siblings on personality will be associated with sibling relationship quality. Observational data on child personality and maternal-report data on sibling relationship quality were collected on 321 sibling dyads (N = 642). Children were videotaped while completing five tasks, and personality traits were rated by independent raters based on thin-slice methodology. Support was found for the absolute value hypothesis but not the sibling similarity hypothesis: the personality traits of younger siblings predicted sibling relationship agonism, particularly when the older sibling was female. Findings suggest that older sisters are more sensitive to negativity in their younger siblings than are older brothers.