Etymology
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sister (n.)
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- (source also of 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).
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step-sister (n.)
also stepsister, mid-15c., from step- + sister (n.).
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sororal (adj.)
1650s, from Latin soror "sister" (see sister) + -al (1).
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sisterly (adj.)
1560s, from sister + -ly (1). Related: Sisterliness.
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sisterhood (n.)
"state of being a sister," late 14c., from sister + -hood. Meaning "a society of sisters" (usually a religious order) is from 1590s; sense of "women having some common characteristic or calling" is from c. 1600.
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sis (n.)
1650s, abbreviated form of sister; in American English, applied generally to girls and young women (1859). It also was the familiar short form of Cecilie, Cicely, a common name for girls in the Middle English period.
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sorority (n.)
1530s, "a society of women, body of women united for some purpose," from Medieval Latin sororitas "sisterhood, of or pertaining to sisters," from Latin soror "sister" (see sister). Sense of "women's society in a college or university" attested by 1887 (Alpha Delta Pi claims founding in 1851).
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bury (v.)

Old English byrgan "to raise a mound, hide, enclose in a grave or tomb, inter," akin to beorgan "to shelter," from Proto-Germanic *burzjan- "protection, shelter" (source also of Old Saxon bergan, Dutch bergen, Old Norse bjarga, Swedish berga, Old High German bergan "protect, shelter, conceal," German bergen, Gothic bairgan "to save, preserve"), from PIE root *bhergh- (1) "to hide, protect." Meaning "cover, conceal from sight" is from 1711. Related: Buried; burying. Burying-ground "cemetery" attested from 1711. Buried treasure is from 1801.

The Old English -y- was a short "oo" sound, like modern French -u-. Under normal circumstances it transformed into Modern English -i- (in bridge, kiss, listen, sister, etc.), but in bury and a few other words (merry, knell) it retained a Kentish change to "e" that took place in the late Old English period. In the West Midlands, meanwhile, the Old English -y- sound persisted, slightly modified over time, giving the standard modern pronunciation of blush, much, church.

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