This is the 1267th most frequent Japanese word.
姉妹
"Sisters."
"姉妹" is used here to refer to a familial relationship, specifically female siblings.
私には二人の姉妹がいます。
I have two sisters.
"姉妹" is used metaphorically here to describe a close relationship between cities.
その町には姉妹都市があります。
That town has a sister city.
"姉妹" designates the relationship between characters as sisters in the story.