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Social structure in human societies is underpinned by the variable expression of ideas about relatedness between different types of kin. We express these ideas through language in our kin terminology: to delineate who is kin and who is not, and to attach meanings to the types of kin labels associated with different individuals. Cross-culturally, there is a regular and restricted range of patterned variation in kin terminologies, and to date, our understanding of this diversity has been hampered by inadequate techniques for dealing with the hierarchical relatedness of languages (Galton’s Problem). Here I use maximum-likelihood and Bayesian phylogenetic comparative methods to begin to tease apart the processes underlying the evolution of kin terminologies in the Austronesian language family, focusing on terms for siblings. I infer (1) the probable ancestral states and (2) evolutionary models of change for the semantic distinctions of relative age (older/younger sibling) and relative sex (same-sex/opposite-sex). Analyses show that early Austronesian languages contained the relative-age, but not the relative-sex distinction; the latter was reconstructed firmly only for the ancestor of Eastern Malayo-Polynesian languages. Both distinctions were best characterized by evolutionary models where the gains and losses of the semantic distinctions were equally likely. A multi-state model of change examined how the relative-sex distinction could be elaborated and found that some transitions in kin terms were not possible: jumps from absence to heavily elaborated were very unlikely, as was piece-wise dismantling of elaborate distinctions. Cultural ideas about what types of kin distinctions are important can be embedded in the semantics of language; using a phylogenetic evolutionary framework we can understand how those distinctions in meaning change through time.