User blog comment:Omega natsu2/Episode 182: The Burning Earth, Review./@comment-24748439-20140517125916/@comment-7418318-20140517184813

Ototo is polite for younger brother, onii-chan is plain for the same.

Just as otousan and chichi are the same, but in different politenes, or kimi and anata or atashi and watashi, ect.

This is how I learned it from my japanese professor who was born and grew up in Japan and only moved to Austria when she was over 30.