also: にいに
noun
Children's language; used by young children to address or refer to their older brother. Equivalent to お兄ちゃん.
にいにい、一緒に遊ぼう！
Big brother, let's play together!
うちのにいにいはサッカーが上手なんだ。
My big brother is good at soccer.
Variant with long vowel marks; emphasizes the drawn-out pronunciation.
Reduplicative baby-talk form derived from 兄 (にい), itself a shortening of お兄さん or similar. The exact origin is uncertain, but it follows a common pattern in children's language.