Midori’s older sister is wearing glasses.
I totally thought you were Viet's older sister, but you aren't, eh?
My older sister thinks that buying a motorcycle will help her get a boyfriend.
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!"
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).