sister

A girl or woman who has sworn loyalty to another despite not being biologically related. Sally and Rita are such good friends, you rarely see one without the other. They're blood sisters.

A female who is older than one or more of her siblings. I'll ask my older sister to help us with the move. Connie's older sister will be there on Saturday, and I'd really like you to meet her. Our neighbor Jill was like an older sister to me when I was growing up.

slang Said among female friends, as a reminder that their friendship is more important than relationships/interactions with men. Come on, don't ditch us for that guy you just met! Sisters before misters!

One's younger female sibling. Yeah, I have a younger sister—her name is Jill. Bella is Johnny's younger sister.

a black person's female, black friend. Many of the top singing groups of the '60s featured soul sisters.

a timid person, usually a male. It looks like Dave is the weak sister on the team. We've got to pull together and stop playing like a bunch of weak sisters.

Someone devoted to charities, or (less charitably) a do-gooder. Originally a newspaper reporter or editor, invariably a woman, whose assignment was to produce sentimental stories and interviews that would appeal to female readers. By extension, the phrase came to mean any overly emotional person, whether male or female, especially one involved in charitable and public service efforts where sad tales of the recipients would tug on their heartstrings.