Responding to recent discussions of brother-sister marriage in Roman Egypt, this article re-examines the Greek and Egyptian evidence for the practice, both papyrological and literary. Exploring possible antecedents in Egypt and Greece and the distinctive development of Egyptian inheritance practice, we argue that the brother-sister marriages involved real siblings, and that by the beginning of Roman rule such marriages were legitimised by a Ptolemaic law and the prevalent belief that they followed ancient Egyptian custom. But new circumstances introduced by Roman rule, particularly the increasing importance of private property ownership, encouraged the practice to become popular through much of northern Egypt. The explanation for brother-sister marriage in Egypt must be sought in the immediate local historical context, not that of the Eastern Mediterranean generally.