TLDR: Older Brother

A little more detail if you like-
Onii can end with 3 different Japanese honourifics, -San, -Chan, or -Sama.
-San is the basic ending for the basic big bro.
-Chan is a more compassionate ending for Onii if you really love your big brother and feel emotionally attached to him greatly
-Sama is a bit of a formal ending for big brother, nothing too special.

Onii-Chan means "Big brother" in Japanese. Most people assume it's a mating call of some sort because of hentai but no that's just hentai. Lots of hentai has incest.

Your older brother who fucks you.

Means sister or brother.

1. Used to refer to your older brother in a cute way (in japanese).
2. In hentai, mostly lolicon, the lolis use it to refer in a cute way to their lover.
3. Cringey term that lots of weeaboos use to refer to their crushes.

To most normal people it means "brother" but...
if you're a nasty, it means "fuck buddy"

The japanese of big brother (or just brother). And please dont say this to japanese ppl unless theyre your brother.