As you may be aware, nepotism is quite prevalent in Spain, and I imagine it is in other Latin countries. In fact, outsiders and not really trusted and in many businesses here in Spain - even in the multinationals like Unilever, General Motors, IBM, where you would have thought a different mindset regulated their employment practices - nepotism is rife, so much so that one's chance of employment is often more regulated by the contacts known, than one's qualifications, experiences, and know-how!
With that said, in light of your hatred for your brother, if you need the job, then take it. It doesn't seem to be your brother's friend you hate, right? So, I can't see the problem.
To the best of your ability, you should not allow family or private mess-ups to interfere with your professional life. You try to leave it at home as best as possible. I know it's not easy, but, still, in this market-world, you've got to man-up.
If you see that your collegue (the brother's mate) is passing on work related issues and work performance behaviour outcomes to other external parties (your brother), you should immediately discuss the matter with him, say it is unpleasent and causing strife in your own family. Unless this mate is a total loon, he or she will listen up and take note. If it persists a step further, you can take action. Record the event and report it to your employees. It is out of order, unprofessional, and in many cases completely illegal. Your working relationship with the mate should be professional at all times.
If your mate is going to be your boss, your employee, then quite simply, I'd avoid the strife, and politely turn down the offer - unless you're starving.
Finally, as jeeprs
has noted you should try to give up on this hatred thing.