User blog comment:Omega natsu2/Episode 182: The Burning Earth, Review./@comment-24748439-20140517125916/@comment-1359322-20140518035549

@Nick actually onii-chan is only for older brother. Unlike the west which use one word "brother" for all, Chinese and Japanese actually divide the terms or specifically Kanji for Older and Younger Brother or sister.

Also on the father thing, the Chichioya or otou-san thing isn't on politeness it's on address, Chichioya is usually use to address your father to a third party, or when a third party address it. In simplier explanation, chichioya (父親) is father, while otou-san is dad.

For example:
 * Dad (Otou-san お父さん) this is my friend. (Talking to father)
 * This is my father (Chichi 父). (Talking to a third party)
 * So that is your father (父親 Chichioya). (Third party referring to your father.