Yes this is a banable offense called multiclienting.
Some people say that it doesn't really matter unless you catch someone doing that in a tourney or something, but nevertheless, please PM a GameMaster for them to handle next time.
Marked for a GM to solve - thank you for this report.
Edit: As for you attempting to withdraw your report because the perpetrator claims that his cousin is playing from the same account, that makes it worse as he is now multiclienting AND account sharing, which is illegal either way.
To reply to your edit; no, it is not OK, and he should be dealt with soon.
Last edited by firebolty; Feb 21, 2012 at 06:27 PM.