A theory (by Norinaga MOTOORI in his essay "Tamakatsuma" (A Jeweled Basket)) states that Kagami no Okimi (鏡王女), who appears in "Manyoshu" and "Nihon Shoki" may be an elder sister of Nukata no Okimi, assuming that '鏡王女' means '女 (daughter of) 鏡王(Kagami no O)', but this may be far-fetched.
『万葉集』『日本書紀』に見える鏡王女（鏡王女）を姉とする説もあるが（本居宣長『玉勝間』）、それは「鏡王女」の表記を「鏡王の女（むすめ）」と解釈したもので、無理があろう。