PS3 Promo Code will activate the game license on your SEN, which at the first launch of the PS3 game client, it will ask for your SQEX ID and which Service Account to tied your SEN to. Once SEN is tied to SQEX and which Service Account, it cannot be unlniked (easily).

This is fine if you are adding PS3 as another platform to your existing SQEX ID and the same Service Account what you use on Windows. If you add as a different Service Account, it is effectively another subscription.

However for your brother to play with your simultaneously, the best/easiest is for him to have his own SQEX ID, and with his copy of the game.