This article examines the cases of children born out of wedlock and adopted children with the aim of depicting the mechanisms through which the concepts of biological fatherhood, derived from the human-rights framework, and adoption, derived from the customary law framework, have been adopted into Indonesian Islamic family law. We argue that the introduction of external concepts into family law pertaining to Muslims requires an adaptation process in which the relation between these external concepts and core Islamic family law concepts is determined. In the case of children born out of wedlock, this adaptation to core Islamic norms means that biological fatherhood does not lead to a full legal father-child relationship, despite a 2012 Constitutional Court ruling establishing that children born out of wedlock have a civil relationship with their biological father. In the case of adoption, it means that there is no full adoption, despite recognition of customary adoptions under Indonesian law. We argue that in a context of strong support for a religion-based family law, reforms tend to take the form of conditions or exceptions to core religious concepts, as replacing these concepts altogether would be perceived as jeopardizing the religious character of the law. While attempts to replace core Islamic family law concepts will inevitably meet strong resistance, there is much more tolerance for introducing family law reforms that aim at changing the way that Islamic concepts are applied in practice.