sister

be (all) brothers/sisters under the skin

Of two or more people, to have thoughts or feelings in common, despite other obvious differences between them. As much as you dislike your chatty new co-worker, she's as nervous and insecure as you are—you're really sisters under the skin.
See also: brother, sister, skin

blood 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.
See also: blood, sister

older sister

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.
See also: older, sister

sisters before misters

slang An expression 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!
See also: before, mister, sister

sob sister

1. A writer or journalist who keeps an advice column in a newspaper to answer readers' problems or quandaries. Collins worked as a sob sister for her local newspaper for several years before moving on to a permanent position with the New York Times.
2. A writer or journalist who focuses on or specializes in overly emotional, dramatic, or sentimental articles. They hired me as a sob sister to write about the plights of those in need of charity around the city.
3. A girl or woman who is prone to overly emotional pleas, complaints, or outbursts. It's a very difficult and narrow path to tread as a woman in the business world. If you show any emotional vulnerability, you are considered a sob sister, but if you allow nothing to show through, you're seen as some stuck-up ice queen—there's just very little in the way of a middle ground. There's this sob sister in my group who just drains all my energy with her constant complaining and whining.
See also: sister, sob

soul sister

1. An African-American woman, especially one who strives to better the lives of other African-Americans. It makes me proud, after so many years of pushing for greater representation in leadership, that a soul sister has finally been elected to our state congress.
2. A woman with whom one shares an intense, intimate, emotional connection. Janet has always been a soul sister to me, and over the course of our 20-year friendship, I feel like I've gotten to know her better than anyone else. When I got my first acting gig at 18, I was just a scared, cocky kid. I connected with Sarah on set, and she became my soul sister, taking me under her wing and teaching me what it meant to be a woman.
See also: sister, soul

weak sister

A weak, spineless, unreliable, or ineffectual person within a group or team. Don't worry, we reassigned him. The last thing we need right now is some weak sister dragging this case down. I knew everyone on the team thought of me as something of a weak sister, so I spent the whole off-season training and practicing.
See also: sister, weak

younger sister

One's younger female sibling. Yeah, I have a younger sister—her name is Jill. Bella is Johnny's younger sister.
See also: sister, young
Farlex Dictionary of Idioms.

(soul) sister

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

weak sister

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.
See also: sister, weak
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.

be (all) brothers/sisters under the ˈskin

be men/women with similar feelings, in spite of outside appearances, position, etc: Actors and politicians are brothers under the skin. They both need public approval.
See also: brother, sister, skin
Farlex Partner Idioms Dictionary

sister

1. n. a (female) friend. (Originally underworld. Sometimes a term of address.) Come here, sister. I gotta have a word with you.
2. n. a fellow sorority member. One of my sisters let me borrow her car.
3. n. a fellow feminist. We can do this thing, sisters, we can do it!
4. Go to (soul) sister.

sob sister

n. a weak woman who is prone to crying. I had another sob sister in the office today. Went through half a box of tissues.
See also: sister, sob

(soul) sister

n. a black person’s female, black friend. (see also sister.) One of the soul sisters dropped by to talk.
See also: sister, soul

sister

verb
See soul sister

weak sister

n. a timid person, usually a male. Another weak sister and we’ll have to quit. We’ve got to pull together.
See also: sister, weak
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions

sob sister

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.
See also: sister, sob
Endangered Phrases by Steven D. Price
See also:
  • be (all) brothers/sisters under the skin
  • be brothers/sisters under the skin
  • read (one's) thoughts
  • at the back of (one's) mind
  • at the back of your mind
  • cut both ways
  • cut both/two ways
  • it cuts both ways
  • it cuts two ways
  • there's no accounting for taste
References in classic literature
'The notion of you among professionals, Amy, is really the last thing I could have conceived!' said her sister. 'Why, how did you ever get here?'
She spoke as if her sister, between two and three years her junior, were her prejudiced grandmother.
'A great many years ago--for the fifteenth century was scarce two years old at the time, and King Henry the Fourth sat upon the throne of England--there dwelt, in the ancient city of York, five maiden sisters, the subjects of my tale.
'But, if the four elder sisters were lovely, how beautiful was the youngest, a fair creature of sixteen!
Miss Crawford was glad to find a family of such consequence so very near them, and not at all displeased either at her sister's early care, or the choice it had fallen on.
"My dear sister," said Mary, "if you can persuade him into anything of the sort, it will be a fresh matter of delight to me to find myself allied to anybody so clever, and I shall only regret that you have not half a dozen daughters to dispose of.
God said that my sister must come first, but He put His hand on my mother's eyes at that moment and she was altered.
"You would like those, Dorothea," said Celia, rather falteringly, beginning to think with wonder that her sister showed some weakness, and also that emeralds would suit her own complexion even better than purple amethysts.
Elinor, persuaded that he had some communication to make in which her sister was concerned, impatiently expected its opening.
Kitty insisted on having her way, and went to stay at her sister's and nursed the children all through the scarlatina, for scarlatina it turned out to be.
"Well, sister, you're late; what's the matter?" said Mrs.
By this time, my sister was quite desperate, so she pounced on Joe, and, taking him by the two whiskers, knocked his head for a little while against the wall behind him: while I sat in the corner, looking guiltily on.
The sisters drew back horror-struck into the furthest corner of the room.
"So I went this morning to Westmor eland House to ask you for my sister's address, and to acknowledge plainly that I suspected you of being again in correspondence with Mrs.
"Well, really, Nancy, just because I happened to have a sister who was silly enough to marry and bring unnecessary children into a world that was already quite full enough, I can't see how I should particularly WANT to have the care of them myself.