Origins
The English word 'sister' has a complex phonological history that reveals the deep impact of Viking-era Scandinavian contact on even the most basic English vocabulary. It derives ultimately from Proto-Indo-European *swésōr, one of the core kinship terms of the proto-language, reconstructed from cognates across nearly every IE branch: Latin 'soror,' Sanskrit 'svásar-,' Avestan 'xvaŋhar-,' Old Church Slavonic 'sestra,' Lithuanian 'sesuo,' Old Irish 'siur,' Welsh 'chwaer,' Armenian 'k'oyr,' and Tocharian B 'ṣer.'
The PIE form *swésōr has been the subject of considerable etymological debate. The most widely accepted analysis derives it from the reflexive pronoun root *swe- (meaning 'self' or 'one's own,' the source of Latin 'suus' and English 'self') combined with a feminine suffix *-sōr, yielding a meaning something like 'woman of one's own group' or 'female of one's own kin.' This would parallel the structure of other PIE kinship terms built from possessive or relational elements. An alternative theory connects the second element to a PIE word for 'woman,' but no such root has been satisfactorily identified.
The Proto-Germanic form was *swestēr, preserved faithfully in Old English 'sweostor.' This form, if it had developed through normal English sound changes alone, would have yielded something like Modern English 'swester' — retaining the initial /sw/ cluster. The actual modern form 'sister' shows the loss of this /w/, a change attributed to the influence of Old Norse 'systir.' During the centuries of Scandinavian settlement in England (roughly 800–1100 CE), the Norse form, which had already lost the /w/ by a regular Norse sound change, influenced the English word's pronunciation. This is not a straightforward borrowing — the word was not replaced wholesale — but rather a case of phonological contamination, where a native word was reshaped under the influence of a closely related cognate from a sister language. The process was likely aided by the fact that the two forms were obviously 'the same word' to bilingual speakers in the Danelaw.
Proto-Indo-European Roots
German 'Schwester' preserves the original /sw/ cluster (spelled 'schw-'), as does the archaic Dutch form. The development of the initial consonant across branches is instructive: PIE *sw- remained in Germanic and Slavic, became /sv-/ in Sanskrit, was simplified to /s-/ in Latin 'soror' (with rhotacism turning the intervocalic *-s- to *-r-), and became /xw-/ in Welsh 'chwaer' and /x-/ in Avestan 'xvaŋhar-.' These regular correspondences confirm the PIE reconstruction.
Latin 'soror' deserves special attention because it appears quite different from the Germanic forms but is demonstrably from the same root. The initial *sw- simplified to *s- in Latin, and the medial *-s- between vowels underwent rhotacism (the Latin sound change where intervocalic /s/ became /r/), producing the double-r form 'soror' from an earlier *sosōr < *swesōr. This Latin form is the source of English 'sororal' (of or relating to a sister), 'sorority' (a sisterhood, especially in the American collegiate sense), and 'sororate' (the anthropological practice of marrying a deceased wife's sister).
In Old English, 'sweostor' carried both the biological meaning and the extended sense of a female member of a religious community — a nun. This usage persists in Modern English, where Catholic and Anglican nuns are addressed as 'Sister,' and the nursing profession adopted 'Sister' as a title for senior nurses in British hospitals. The metaphorical extension from kinship to professional solidarity mirrors the parallel development of 'brother' for monks and friars.
Old English Period
The Old English plural of 'sweostor' was unchanged or took the form 'sweostor' (an old r-stem pattern), but this was regularized to 'sisters' by Middle English. Unlike 'brother/brethren,' no archaic plural survives for 'sister' — there is no *'sistren' in standard English, though the form 'sistren' does appear in some Caribbean English dialects, formed by analogy with 'brethren.'
The word's cultural productivity in English is extensive. 'Sisterhood' mirrors 'brotherhood' as a term for female solidarity and collective identity. 'Sister city' (or 'twin city') extends the kinship metaphor to municipal relationships. In African American Vernacular English, 'sister' (like 'brother') serves as a term of community solidarity. The feminist movement adopted 'sisterhood' as a central concept — Robin Morgan's 1970 anthology 'Sisterhood Is Powerful' made the term iconic — drawing on the same ancient metaphorical extension from biological kinship to chosen solidarity that has characterized the word since PIE times.