お兄さん
おにいさん · oniisan · JLPT N5 · noun
brother
お兄さん = polite o + 兄 nii (older brother) + san (Mr/Ms)
The core is 兄 (nii), older brother. Wrap it in two politeness markers: honorific お in front and さん behind. That double padding is why it suits someone else's brother; your own is plain 兄 (ani).