For many individuals, religious development begins in the family. Previous literature has confirmed this assumption and demonstrated that parents play a dominant role in the development process. There is a gap in the current literature regarding how other family members could also potentially contribute to religious development. This project takes the first step towards understanding how siblings may influence religious development by investigating the extent to which siblings share similar religiosity levels. Hierarchical linear models are applied to data from the National Longitudinal Study of Adolescent Health in order to assess whether siblings share similar religiosity levels. This project also investigates potential explanations of this similarity. The results of this thesis demonstrate that siblings share common levels of religiosity. The similarity was still moderate after controlling for known predictors of adolescent religiosity such as parental influence, religious affiliation, race, and age. Sibling communication also explained a small proportion of sibling religious similarity. The results of this project leave open the possibility that siblings may contribute to one another’s religious development. Suggestions for future research are discussed.