Etymology
1. Etymology tree
2. Proto-Indo-European *swé
3. Proto-Indo-European *h₁ésh₂r̥
4. Proto-Indo-European *su-h₁ésh₂-ōr?
5. Proto-Indo-European *swé
6. Proto-Indo-European *-sōr
7. ?
8. Proto-Indo-European *swésōrder.
9. Proto-Germanic *swestēr
10. Proto-West Germanic *swester
11. Old English sweostor
12. Middle English suster
13. English sister
14. Inherited from Middle English suster, from Old English sweostor, from Proto-West Germanic *swester, from Proto-Germanic *swestēr, from Proto-Indo-European *swésōr.
15. Doublet of soror. 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”).
16. In standard English, the form with i is due to contamination with Old Norse systir (“sister”).
17. The plural sistren is from Middle English sistren, a variant plural of sister, suster (“sister”); compare brethren.
18. The sense for "Adelpha-genus butterfly" is a semantic loan from translingual Adelpha, itself from Ancient Greek ἀδελφή (adelphḗ, “sister”). See: Latin '
soror'.