If none of the workforce has an employment contract or a union membership, then everyone's an "at will" employee and therefore can be fired without notice at any time for any reason or no reason at all, and everyone can quit without notice and for any reason or no reason. Likewise, the employer can do less than fire someone, like demote them, reduce their hours or salary, or punish thejm for their brother's error. There's no law against nepotism, unfortunately, it's just part of "office politics."
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Your choices are to quit, learn to live with the fact that the brother is going to get preferred treatment or go over the main bosses head and complain to the owner, if he is not the main boss.
The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response does not form an attorney-client relationship, nor is it intended to be anything other than the educated opinion of the author. It should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response might possibly change.