Did you mean 14 by any chance, because I would think that at twenty four you have no obligation, or even right to censor him at all!
Grand theft auto is a very sticky situation. I have played it myself and personally find myself at odds with myself. On the one hand its a game, and not real, and I would never do any of the activities there in the real world.
On the other hand, simulated exploitation, being a drug dealer, pimp, murderer and thief, even in a simulated environment cant be good. The vicarious enjoyment that is derived out of the game is symptematic of latent desire to sin that is present in the human condition post fall.
I dont think our proclivity to sin should be denied or ignored, but neither should it embraced or encouraged. It ought to be acknowledged, analysed and confronted.
I personally dont know what to say to you. I have played these games myself, and found myself enjoying them, until later, during prayer did i consider the reasons why I found them enjoyable.
If your brother is 24: butt out and let him make up his own mind, your pro and fair enough, he might be too which is fine, but he might be against it and that is ok too.
If your brother is 14: he is too young to be exposed to some of the content of game, never mind participating in it, even virtually.
maybe pray for guidance? Ill say one for you too!