Meaning of “sister” - Learner’s Dictionary

sister

noun [ C ] uk us /ˈsɪstər/
RELATIVE

A1 a girl or woman who has the same parents as you:

an older/younger sister
my big/little sister
RELIGION (also Sister)

a nun (= woman who lives in a female religious group):

Sister Bridget
NURSE (also Sister)

a female nurse in the UK who is responsible for a hospital ward (= an area of a hospital containing beds for sick people):

I asked Sister if he would be able to go home soon.
MEMBER

a woman who is a member of the same race, religious group, organization, etc

(Definition of “sister” from the Cambridge Learner’s Dictionary © Cambridge University Press)