honorific suffix used after the name of a woman of equal or higher status
「姉」 has other readings with different meanings.
花子姉のところに遊びに行った。
I went to visit sister Hanako.
美和姉はいつも優しくて困ったときに頼りになる。
Sister Miwa is always kind and reliable when you're in trouble.
隣の雪姉にお菓子をもらったよ。
I got sweets from sister Yuki next door.