Ok there is 2 ways you can handle this.

1. You have already linked your SE account to your PSN account basically meaning its your copy - SE don't like unlinking PSN accounts and you will have a fight on your hands.
2. Already a question you asked and here is an answer - Your brother makes a psn account - buy the game and link his psn account to his SE account and its sorted except of course being a little out of pocket due to buying 2 copies.

You can have as many people as you want playing FFXIV on one PS4 as long as they have bought there own copy and link there SE account to there PSN account. None will affect the others.