Your sister is another girl who is born to the same parents to whom you are born.(noun)
An example of sister is a girl who is born to your parents a year after you are born.
See sister in Webster's New World College Dictionary
Origin: ME < ON systir (akin to OE sweoster > dial. suster), akin to Ger schwester < IE *swesor-, sister (> Sans svasar, L soror, OIr siur) < *sewe-, (one's) own, refl. pron. (see suicide) + *sor-, woman: hence, lit., woman of our (family)
See sister in American Heritage Dictionary 4
Origin: Middle English
Origin: , from Old Norse systir; see swesor- in Indo-European roots.
Learn more about sister