お兄さん(onii san) or 兄さん(nii san) is somones brother (like talking to persons brother)
兄(ani) is the "my" brother (the speaker brother)
if I talk about the my brother I say "ani", if I talk about the someones brother I say "onii san" or "nii san" it is the polite~ ("onii san" is the more polite)
and if I talk to my brother or call my brother I say "nii san" too. Because it is the polite. but I also can say "nii chan" or something (＾～＾) and sometimes I use the "O" too.
the same is with other family member
またね！
ミルキィ・ドゥリーム
milky-dream