1Sisters — refers to a female sibling relationship (older and younger sisters together), and is also used figuratively for paired or affiliated entities, as in "sister school" or "sister city."
うちは三人姉妹で、私は真ん中です。
There are three sisters in my family, and I'm the middle one.
あの二人はよく似ているけど、姉妹ではなく親友なんです。
Those two look alike, but they're not sisters — they're best friends.
うちの大学はアメリカの大学と姉妹校の関係を結んでいる。
Our university has a sister-school relationship with a university in America.