Thread:JouXIII/@comment-25383025-20171018153113/@comment-26369387-20171018175142

Well, it doesn't change the fact that 兄弟, kyōdai can be translated as "brothers" and "siblings"(although more of as "brothers", I think; for example, Erlic Brothers in Fullmetal Alchemist are called エルリック兄弟, Erurikku Kyōdai in Japanese). Also, 兄弟, kyōdai refers to both brothers/siblings, not just the other(meaning your translation of 鬼兄弟 as "Demon Sibling" is kinda wrong).

However, since you mentioned twins, there is option I think you could go with: 霧隠れの鬼双児, Kirigakure no Oni Sōji(Demon Twins of the Hidden Mist). As for Sengetsu's personal epithet, I would go something like 霧隠れの蒼鬼, Kirigakure no Sōki(Pale Demon of the Hidden Mist), since Sengetsu seems to pretty pale skin. For twin sister, maybe 霧隠れの鬼女, Kirigakure no Kijo(Demoness of the Hidden Mist)?