a girl or woman who has the same parents as another person:
Sophie and Emily are sisters.
Emily is Sophie's younger/little/older/big sister.
a girl or woman who treats you in the kind way that a sister would:
Lynn's such a good friend - she's like a sister to me.
a woman who shares an interest with you, especially that of improving women's rights:
[ as form of address ] "We must continue the fight, sisters!"
used to address a woman:
[ as form of address ] OK, sister, move it!
a female member of a religious group, especially a nun:
Sister Bernadette
[ as form of address ] God bless you, Sister.
a female nurse who is in charge of a department of a hospital:
The sister told us that visiting hours were over.
[ as form of address ] Excuse me, Sister, could I have some water?