日語 Japanese Ask Questions Thread2 質問

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Japanese - Ask questions thread
http://dis.4chan.org/read/lang/1174719097/l50

Seeking definitions for oni, onisan, onesan, oneesan, oniisan.  If oni means demon (as I had thought to believe), how does onisan mean older brother?  Or am I reading something wrong?  Keep in mind I'm talking about romanizations.  Thank you.