I think that within the game, not knowing who is playing who, if this sort of thing occurs between characters, it is not only ok, but actually a good thing.

For example: Vashka posted something on the Battlefield about his closeness to Kalmah and to Xandrya. He felt that they were like the brother and sister he never had. Zekasiq and Aneriasha developed a close relationship that started out as a romance and ended up as a lesbian relationship. Sylvrin felt a real closeness to Zekasiq as they interacted together and she often referred to Zekasiq as her brother/sister depending on Zek's gender at the time (see battlefield posts for details.) "Best friends" are mentioned many many times in the Battlefield posts. That kind of close relationship can and often does develop between characters within the game. Cabals can also
encourage this sort of thing to happen.

The difficulty with what you're suggesting is that the relationship would be pre-planned OOC. This detracts from the RP sponteneity for one thing and can actually interfere with the way you want to develop your character...especially if you discuss the details of your character development outside the game. You would know who is playing your brother and could use that to your advantage in ranking and PK situations as well. I'm not saying that you necessarily would do that...only that it would be VERY TEMPTING. And anyone who saw that the two "brothers" were always together would assume that you are permagrouping and cheating in some way...even if you aren't. That could be very detrimental to you in the long run.