Oniisan O+san=very respectful way

Oniichan =respectul and very affectionate

Niisan 

Niichan 

Chan is used in an affectionate way, San in a respectful and formal way,

Aniki, is almost a slang like, something like English - dude - bro- , though this also can be used in an affectionate way but still it is a bit rough style. A female person who uses this is considered to be rather masculine type. Yet it does not mean that you are being rude. 

wow! i see, well noted and arigatou gozaimasu Roo san^^