'Sister' is PIE *swesor — possibly 'woman of one's own kin.' Shaped by Old Norse 'systir.'
A female sibling; a woman or girl in relation to other children of the same parents.
From Old English 'sweostor' (later 'suster' by Norse influence), from Proto-Germanic *swestēr, from PIE *swésōr. The PIE form is securely reconstructed from Latin 'soror,' Sanskrit 'svásar-,' Old Church Slavonic 'sestra,' and others. The etymology of *swésōr itself is debated: one theory derives it from *swe- (self, one's own) plus *-sōr (woman), meaning 'woman of one's own kin
The Modern English form 'sister' rather than the expected 'swester' is due to Old Norse influence — the Norse form 'systir' lacked the initial /w/ cluster, and Viking-era contact reshaped the English word, making 'sister' one of the many everyday English words subtly altered by Scandinavian settlers.