Can your boss make his supervisor's come in on weekends & not get paid or added time for it? Is this legal & what can you do about it? What if the Internal Affair's guy is friends with your boss, how is this fair to the employees having problems with that boss?

Is there a union? Is there an employment contract?

What you can do depends on the State, whether the employee is salaried or hourly. You can always contact Wage and Hour (or whatever it's called in your State) and ask.

If employment is at will and a person can be fired for anything (or nothing) I'd be cautious.

Being a supervisor would indicate an exempt employee. So you your boss could legally require you come in on weekends.

As for the IA thing, that's something you would have to bring up with upper management or HR.