Older Sister, Miss
noun
The お makes the word more polite. You're elevating the "older sister" title because you're a good younger sibling. Otherwise, the meaning of this word is the same as the kanji - just the feeling is different. You can also use this word to refer to a youthful-looking woman that you don't know, as in "Excuse me miss!"
おねえさん
The reading is different too. It has some hiragana attached to it, which is a clue that this uses a kun'yomi reading. Let's use a mnemonic to learn that reading. Imagine your older sister (even if you don't have one). She is, for some reason, a horse. Imagine petting her, giving her carrots, and doing other horse-related things with her. What do horses say? They say nehhhh (ねえ).
あの人が、レイチェルのお姉さん？
Is that person Rachel's older sister?
この子は、私のお姉さんの子です。
This child is my sister's child.
お姉さん、出口はあっちですよ。
Miss, the exit is over there.