Nepotism only works when the person who benefits does a good job.
For the most part, people do not complain about those people who got the role through who they know if they put in the effort and show themselves to be capable at what they do. What truly adds insult to injury is if they turn out to be incompetent. Many business owners see employing family members as a no-brainer, but their coworkers may beg to differ. They can be blind to their flaws as an employee.
Being the person who has to work with an underperforming nepo hire is a stressful experience. Not only are you forced to rectify their mistakes, but speaking up about them can put you in trouble, instead of easing your workload. More often than not, employees in this situation find themselves treading very carefully so as not to upset the boss, while also overextending themselves to make up for their colleague. It is an issue that can only go on for so long.
Taking action in this scenario is usually what is in the best interests of the business, but it has to be done with tact. If not, things can end up worse than when they started.