Look at what playing brothers means:

You are probably talking about a tight knit group (even if you aren't grouped) who can rely on each other.

Unfortunately, no one else gets this without risking betrayal etc.

It's good that you asked here rather than just do it and whine if you get slapped down for it, but a general rule is that you shouldn't know who is playing who (apart from elf bards who are all played by Gorach or a clone of Gorach created specifically to increase the number of elf bards). If you don't know who is playing who, then you wouldn't know for these guys to be your brothers. Ultimately, the important thing is that knowing who plays who shouldn't affect how you interact with them. (ie pk your ooc friends just like they were anyone else.)