sister; female sibling
Refers to a female sibling, but in ordinary Japanese 姉 or 妹 is usually used when specifying one's own older or younger sister. シスター feels English-like, translated, or context-dependent in this sense.
In this movie, there is a scene where the main character calls his older sister his sister.
female partner in a homosexual relationship
Specialized slang use for a female partner in a homosexual relationship. It is not the ordinary general word for a girlfriend or partner.
See also: Ｓ (エス)
In older slang, a partner in a relationship between women may be called a sister.
After graduating from university, she became a nun.
That woman is called 'Sister' by everyone.