SISTER, the correlative of brother (q.v.), a female in her relation to the other children born of the same parents, also one who has acquired such relationship by marriage, a sister-in-law, or by adoption. The O. Eng. word was sweostor; cf. Dutch zuster, Ger. Schwester, Goth, swislar; in M. Eng. this appears as suster; the Scandinavian form appears in Icel. systir, Swed. systor, Dan. sbstor, and this has curiously taken the place of the true English form suster. Outside Teut. are found Lat. soror for sosor, Skt. svasti; the origin is not known, but it may be related with Skt. svasti, happiness, joy. The Lat. consobrinus, which has given " cousin," is from con-sobrinus, sosbrinus, from the stem of soror, sister. Ay " brother " and " brethren " are used for the male members of a religious body or community, so also is " sister " for the female members; more particularly it is applied to the members of a female religious order or community, a " sisterhood," in the Roman and other churches, who are devoted to a religious life, works of charity or mercy, whether bound by irrevocable vows or not.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)