kinship terms encode relative age (姉/妹, 兄/弟)

Japanese kinship terms always grammaticalize relative age within the same generation: 姉 (older sister) vs 妹 (younger sister), 兄 (older brother) vs 弟 (younger brother). There is no general word for 'sister' or 'brother' that ignores age — the speaker must commit to which one. Reflects the broader cultural emphasis on hierarchy and seniority within family relationships.