It is very natural to assume that almost all languages of the world have the kinship terms for the siblings of the same blood. Generally sex and age are the main factors to determine the terms for the siblings. It is possible to classify eight types of the sibling terms on the basis of sex and age of the alter. Among the eight types, however, only three are very common and the other five types are logically possible but rare in reality. In this paper, the author tries to postulate two universal principles to determine the types of sibling terms : (a) Sex has priority over age. (b) Less terms are used for the younger alters than the elders. Though the Korean kinship system of the siblings acts up to the principles, it shows some peculiarities compared to the systems of other languages. While most languages use the factors of sex and age of the alters to divide the relations of siblings, Korean adds the sex of the ego to these factors. If the ego and the alter are of the same sex, two terms on the basis of age are necessary to refer to the younger and the elder alters in the Middle Korean. On the contrary, if the ego and the alter are of the different sex, the heterosexuality functions as a distinctive factor and only one term is used to the alters, for whom the factor of age doesn't work. The factors affecting the system of sibling terms have changed from the Middle Korean and the homosexuality of ego and alter doesn't work any longer in the contemporary Korean. In addition to this change, the semantic shifts of dongsaeng(sibling), nui(sister) and orabi(brother) also contribute to build the new sibling system of Korean.