Unless you are an independent contractor with an employment contract or working under a union contract, you are considered an at will employee. At will employees can be terminated/disciplined for anything EXCEPT the set discrimination bases. Under Federal law the discrimination bases are: age, sex, race, color, nationality, religion, disability, marital or military status. (See www.eeoc.gov). Your state laws also enfore these discrimination bases and MAY add one or two of their own. The most common being sexual oreintation.
Harassment, abuse, or hostility in the workplace must be connected to one of the discrimination bases. You will notice that nepotisim and favoritism are NOT discrimination bases. That means that the employer can hire and promote and protect his/her family and friends ---- AS LONG as they DO NOT discriminate againist others along the recognized bases. Go to www.eeoc.org for an understanding of discrimination.