sister

1 :  a female who has one or both parents in common with another

2 often capitalized a :  a member of a women's religious order (as of nuns or deaconesses); especially :  one of a Roman Catholic congregation under simple vowsb :  a girl or woman who is a member of a Christian church

3a :  a girl or woman regarded as a comradeb :  a girl or woman who shares with another a common national or racial origin; especially :  a black girl or woman

4 :  one that is closely similar to or associated with another sister cities

5 chiefly British :  nurse

6a :  girl, womanb :  person —usually used in the phrase weak sister

7 :  a member of a sorority