A female person having the same parents as another person.
M.E. suster, sister, from O.E. sweostor, swuster or Old Norse systir, in either case ultimately from PIE *swesor; cf. Skt. svásar-, Av. xvaηnhar-, Pers. xâhar, xwâhar, as below, Gk. eor, L. soror (Fr. sœur), O.C.S., Rus. sestra, O.Ir. siur, Sw. syster, Dan. soster, O.Sax. swestar, M.Du. suster, Du. zuster, O.H.G. swester, Ger. Schwester.
Xâhar, xwâhar, from Mid.Pers. xwah(ar) "sister;" Av. xvaηnhar- "sister;" cf. Khotanese hvar- "sister;" cognate with E. sister, as above.