Proto-Indo-European Roots

Root/Stem: *swesór-
Meaning:  a sister
Hellenic Greek heor, eor (a daughter, a girl)
Italic Latin soror (a sister) < *sesor
French soeur
Celtic Common Celtic *svesór, > 
Old Irish siur (a sister), gen. sethar, Welsh chwaer, Breton c'hoar, Irish Gaelic deir-fiur (a sister), Scottish Gaelic piuthar, Old Cornish huir, Cornish hoer
Indic Sanskrit svasar- (a sister)
Dardic & Nuristani Khowar ispt-sar, ispu-sar (younger sister) ?
Iranian Avestan xwanghar- (a sister)
Tocharian Tocharian A, B (a sister)
Armenian khoir (a sister), pl. khorkh
Germanic Common Germanic *swistér, *swistór, > 
Gothic swistar, Old English sweostor , Old High German swester,  Old Norse systir, Old Saxon & Old Swedish swiri (son of mother's sister); 
German Schwester, Dutch zuster
Baltic Lithuanian sesuo, gen. seseris, Old Prussian swestro
Slavic Common Slavic *sestra, > 
Ukrainian & Belorussian & Old Russian & Old Church Slavonic & Bulgarian & Serbo-Croatian & Slovene & Czech & Slovak & Russian sestra, Polish siostra, Upper Sorbian sotra
Notes: Though it is one of the typical r-stem nouns, denoting kinship, its case is quite peculiar. Its suffix *-sor can be found in many other words for feminine animate nouns, like Hittite nasu-sara (a queen), Sanskrit strí 'woman', etc. This suffix was here added to the word *swe- which is obviously the reflexive Indo-European pronoun.