'sister' originates from Old English, specifically the word 'sweostor', where Proto-Germanic '*swestēr' and Proto-Indo-European '*swésōr' meant 'sister'.

'sister' changed from Old English 'sweostor' to Middle English forms such as 'suster' and eventually became the modern English word 'sister'.

Initially, it meant 'female sibling', and over time it has largely retained that meaning, though it also developed extended uses for nuns and women in the same organization or group.