Sister
Old English, of Germanic origin; related to Dutch zuster and German Schwester, from an Indo-European root shared by Latin soror .
wiktionary
From Middle English sister, suster, from Old English swustor, sweoster, sweostor(“sister, nun”); from Proto-Germanic *swestēr(“sister”), from Proto-Indo-European *swésōr(“sister”).
Cognate with Scots sister, syster(“sister”), West Frisian sus, suster(“sister”), Dutch zuster(“sister”), German Schwester(“sister”), Norwegian Bokmål søster(“sister”), Norwegian Nynorsk and Swedish syster(“sister”), Icelandic systir(“sister”), Gothic 𐍃𐍅𐌹𐍃𐍄𐌰𐍂( swistar, “sister”), Latin soror(“sister”), Russian сестра́(sestrá, “sister”), Lithuanian sesuo(“sister”), Albanian vajzë(“girl, maiden”), Sanskrit स्वसृ(svásṛ, “sister”), Persian خواهر (xâhar, “sister”).
In standard English, the form with i is due to contamination with Old Norse systir(“sister”).
The plural sistren is from Middle English sistren, a variant plural of sister, suster(“sister”); compare brethren.
etymonline
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).
