older brother; elder brother
「兄」 has other readings with different meanings.
にいちゃん、宿題を教えてよ。
Hey bro, help me with my homework.
隣のにいちゃんはいつも優しくしてくれる。
The older boy next door is always nice to me.
にいちゃんが先に帰ったから一人で遊んでいた。
My big brother went home first so I was playing alone.