Guide for the Perplexed, by Moses Maimonides, Friedländer tr. , at sacred-texts.com
THE two Hebrew nouns ish and ishshah were originally employed to designate the "male and female" of human beings, but were afterwards applied to the "male and female" of the other species of the animal creation. For instance, we read, "Of every clean beast thou shalt take to thee by sevens," ish ve-ishto (Gen. Vii. 2), in the same sense as ish ve-ishshah, "male and female." The term zakar u-nekebah was afterwards applied to anything designed and prepared for union with another object Thus we read, "The five curtains shall be coupled together, one (ishshah) to the other" (aḥotah) (Exod. xxvi. 3).
It will easily be seen that the Hebrew equivalents for "brother and sister" are likewise treated as homonyms, and used, in a figurative sense, like ish and ishshah.