means a female sibling who shares one or both of your parents, or by extension a woman bonded to you as close as family.

from From Old English 'sweostor,' descended from Proto-Germanic 'swestēr' and traceable all the way back to Proto-Indo-European '*swésōr' — one of the oldest, most stable words humans have. Its cousins crowd the family tree: Latin 'soror' (giving us 'sorority'), Sanskrit 'svasar,' German 'Schwester.' The modern form with its hard 'st' was nudged along by the Old Norse 'systir' brought by Viking settlers, the linguistic equivalent of a relative moving in whether you liked it or not.