Depends on the age of the perpetrator and what charges are brought based on how out of hand it gets, who brings in illegal drugs or a gun, takes inappropriate photographs, etc. Read up on Pandora's box.
Under 21 does not mean that you will not be charged as an adult. You could also get your brother evicted. When he has to move back home, he will be quite angry.
Every legal matter is fact specific, and there are often nuances in every case. This is intended for comment only, and does not create an attorney client relationship.
You could get arrested for unlawfully dealing with a child and endangering the welfare of a child.
I am a former federal and State prosecutor and have been doing criminal defense work for over 17 years. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012 and 2013. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. Martindale-Hubbell has given me its highest rating - AV Preeminent - in the areas of Criminal Law, Personal Injury, and Litigation. According to Martindale-Hubbell”AV Preeminent is a significant rating accomplishment - a testament to the fact that a lawyer's peers rank him or her at the highest level of professional excellence." Fewer than 8% of attorneys achieve an AV Preeminent rating. I also have the highest ranking – “superb” – on Avvo. The above answer, and any follow up comments or emails is for informational purposes only and not meant as legal advice.
What might happen is you could be arrested for any of a variety of criminal offenses, and be sued if someone is hurt. And this electronic posting, which can easily be traced back to you or your family, could be obtained by authorities as proof that you were warned how dangerous and criminal and outright stupid it would be to do something like this.
So do not do it, for your sake and for anyone who cares about you.
The response I have provided is general in nature, and does not create an attorney-client relationship. My practice is based in Rhode Island, and the law and practice in other states or jurisdictions may be different.