If I'm not misunderstanding what I just read => http://support.jp.square-enix.com/fa...5&id=5382&la=1

then technically you can okay with 2 characters simultaneously because you have 2 service accounts (1 for ps3 and 1 for PC).

So your brother should be able to play using your account on the PS3 while you are playing on your PC. You may be limited however because of the subscription you have. So like, if you can only create 1 character per server, both of you would have to play on separate servers. But if you have the subscription that allows up to 8 characters then you can both play on the same server together.

Hopefully I'm not wrong. But if I am, I'm sorry