Siblings are important sources of social influence throughout childhood and adolescence. Nevertheless, the processes by which siblings influence one another remain relatively unexplored. We highlight two theories of sibling influence—sibling deidentification and social learning—that offer insights as to how and why siblings develop similar and different attributes, attitudes, and behaviors. Recognizing the need to move past post hoc explanations, we suggest several directions for how these two influence processes can be measured directly in future work. Research on sibling influence also can be improved by integrating these theories and attending to their domains of influence.