Webster's English Dictionary

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sister

sis.ter \'sis-t*r\ n [ME suster, sister, partly fr. OE sweostor and partly 
   of Scan] often attrib d origin; akin to ON systir sister; akin to L soror 
   sister 1a1: a female human being related to another person having the same 
   parents  1a2: HALF SISTER  1a3: SISTER-IN-LAW  1b: a female of a lower 
   animal in relation to another having a common paren t often cap  2a: a 
   member of a religious sisterhood; specif : one of a Roman Cat holic 
   congregation under simple vows 2b: a female member of a Christian church  
   3a: a woman related to another by a common tie or interest  3b: one having 
   similar characteristics to another {~ ships}  chiefly Brit  4: NURSE  5a: 
   GIRL, WOMAN  5b: PERSON - usu. used in the phrase weak sister