Is it different between nii-Chan and oni-Chan?

From what I understand Nii-chan is a more familiar version of Oni-chan. I think it's close to English Big Bro and Big Brother.

... 兄(anii/nii) is how you call your older brother , お兄さん （onii-san) has the sufix 'o' and thats because its not _her_ older brother. if I'm correct

To put it simply, niichan is what u call ur own brother...onichan is just a polite form of calling a male who is older than you... Usually to strangers.....