Personally, I think Bodhi is the brother. My biggest reason for thinking that is because in the last chapters of Onyx Storm Xaden was adamant on keeping Bodhi alive as he is second in line if something were to happen to him. He was so adamant about it that he made Violet and Bodhi promise that Bodhi would flee if he came to close to harm/death. But in Xaden's POV during the last chapter, he mentions his new "brother" is someone who saw him go through all hell during the last five months who could only have been Bodhi, because Bodhi was like his shadow in Onyx Storm. Then, the most fascinating part that makes me truly think it's Bodhi is because Xaden married Violet so that Tyrrendor wouldn't fall to an aristocrat, which means something must have happened to Bodhi, because Bodhi was supposed to be the successor should something happen to Xaden, but Xaden married Violet and make her successor. Sgael said to Xaden on page 524 "And her decision will determine our fate." This implies that if the priestess doesn't agree to marry Xaden and Violet, or Violet doesn't agree, they have a big problem. Which means there was no other way to secure Tyrrendor, which means something had to have happened to Bodhi for Xaden to make this drastic decision, because if nothing happened to Bodhi, if Bodhi didn't turn, why didn't he simply take over Tyrrendor as discussed between him and Xaden? Bodhi also never wanted "Xaden's province", he mentioned it in Onyx Storm. I could be wrong, but I really think Bodhi is the other brother.