In many group-living animals with kin-biased behaviour, individuals discriminate closely related from distantly related conspecifics. To assess relatedness, individuals commonly learn a kin template through prior associations. However, the degree and determinants of flexibility in kin recognition systems remain unclear, although it is known that individuals can change their kin response depending on context and ontogeny. This study evaluates the plasticity of kin discrimination in tadpoles of the frog Rana ornativentris. We raised tadpoles under two different sibship conditions: the pure “P-line”, comprising only siblings and the mixed “M-line”, comprising both siblings and non-siblings. The association preference by a subject tadpole to unfamiliar tadpoles was assessed through binary-choice tests between related and unrelated stimuli. We observed kin bias based on phenotype matching in R. ornativentris tadpoles only when the subjects were small and reared in M-line, and when the stimuli were small. Thus, rearing with both kin and non-kin enhanced kin discrimination. Related experimentation suggested that rearing conditions also affect size preference among non-relatives; i.e., tadpoles from the P-line exhibited a preference for small non-siblings rather than large non-siblings, whereas tadpoles from M-line exhibited no such preference. Taken together, we report a novel finding that variation in prior association modulates the own template for assessing conspecifics, and later shapes both kin discrimination and possibly size-related preference among non-relatives in this species.