An Etymological Dictionary of the German Language/Annotated/Schwester
Schwester, feminine, ‘sister,’ from Middle High German swëster, Old High German swëster, feminine; a common Teutonic and also primitively Aryan word. Compare Gothic swistar, Old Icelandic syster, Anglo-Saxon sweostor, English sister, Dutch zuster, Old Saxon swëstar. The common Teutonic stem swestr-, originated in Aryan swesr- (compare Strom for the insertion of t in sr), nominative singular swésô; compare Sanscrit svasr-, nominative singular svasâ, Latin soror for *swesô-r, Old Slovenian sestra, Lithuanian sesů (for *swesô). The originally meaning of the cognates, as in the case of Bruder, Aryan bhrãtõ (brãhtṛ), cannot be discovered; yet Schwäher, Schwieger, and Schwager (Aryan swekuros, swekrũ, swékrós) are similar in sound, so too Old Icelandic swiljar, ‘husbands of two sisters,’ Old Saxon swiri, ‘nephew, sister's child’ (Aryan swesjo-?), &c., whose common component swe- signified ‘own, his,’ according to Sanscrit sva, Latin suus; Compare sein. For the Aryan terms of consanguinity compare Vater, Mutter, &c.