The ordinary meaning of is younger sister, but in ancient time, it meand beloved girl, thus meant a couple in love and then wife and husband. Nowadays this.
弟, おとうと, otouto, Younger Brother. 16. 妹, いもうと, imouto, Younger Sister. 17. 夫婦, ふうふ, fuufu, Married Couple / Husband and Wife. 18. 主人, しゅじん.