お姉さん
おねえさん · oneesan · JLPT N5 · noun
sister
👧 お + 姉 (older sister) + さん = honorific o + big sis + polite san
The core is 姉 (ane/nee), older sister. Polite お goes in front and respectful さん behind, the same wrapper as お兄さん (older brother) and お母さん (mother). The long ee in o-nee-san matters: it is two beats, not one.