Although the multiracial population is currently the fastest growing racial group in the United States, little remains known about their identity and mental wellness. The purpose of this study was to investigate factors that progress multiracial identity development. More specifically, this study examined the relationship of number of multiracial siblings, sibling phenotype similarity, and sibling relationship quality with the multiracial identity development of multiracial adults. A total sample of 563 multiracial participants were recruited from across the United States and completed an online survey involving self-report questions. The outcome variable of multiracial identity development was measured using the Multiracial Identity Integration Scale. Number of multiracial siblings was measured by a single item on the demographic questionnaire, sibling phenotype similarity was measured by an 11-item scale that the researcher created, and sibling relationship quality was measured by the Lifespan Sibling Relationship Scale. A multiple linear regression analysis and one-way analysis of variance were utilized by the researcher to examine the relationship of the predictor variables with multiracial identity development. The results indicated that none of the predictor variables were found to significantly influence multiracial identity development. Implications of this study include the need for a noticeable increase in research on this identity, counselor trainings and teachings on this population, and the awareness of negative stereotypes about this population still embedded in mental health research and practice. This study provides a starting place for future studies to build upon when investigating multiracial siblings and factors that influence multiracial identity development.