sisteryoudaoicibaDictYouDict[sister 词源字典]
sister: [OE] Sister is one of a widespread family of ‘sister’-words that go back ultimately to Indo- European *swesor. Amongst its other descendants are Latin soror (source of French soeur, Italian sorella, and Romanian sora, not to mention English sorority [16]), Russian, Czech, and Serbo-Croat sestra, Polish siostra, Welsh chwaer, Breton c’hoar, Lithuanian sesuo, and Sanskrit svasar-. To prehistoric Germanic it contributed *swestr, which has evolved into German schwester, Dutch zuster, Swedish syster, Danish søster, and English sister. English cousin goes back ultimately to a compound based on *swesor, the Old Latin antecedent of soror.
=> sorority[sister etymology, sister origin, 英语词源]
sister (n.)youdaoicibaDictYouDict
mid-13c., from Old English sweostor, swuster "sister," or a Scandinavian cognate (Old Norse systir, Swedish syster, Danish søster), in either case from Proto-Germanic *swestr- (cognates: Old Saxon swestar, Old Frisian swester, Middle Dutch suster, Dutch zuster, Old High German swester, German Schwester, Gothic swistar).

These are from PIE *swesor, one of the most persistent and unchanging PIE root words, recognizable in almost every modern Indo-European language (Sanskrit svasar-, Avestan shanhar-, Latin soror, Old Church Slavonic, Russian sestra, Lithuanian sesuo, Old Irish siur, Welsh chwaer, Greek eor). French soeur "a sister" (11c., instead of *sereur) is directly from Latin soror, a rare case of a borrowing from the nominative case.

According to Klein's sources, probably from PIE roots *swe- "one's own" + *ser- "woman." For vowel evolution, see bury. Used of nuns in Old English; of a woman in general from 1906; of a black woman from 1926; and in the sense of "fellow feminist" from 1912. Meaning "female fellow-Christian" is from mid-15c. Sister act "variety act by two or more sisters" is from vaudeville (1908).