I agree with you but I don't want Raí to meet him if it is his brother bc I feel like it would make him sad and Raí being sad makes me sad and noooo I just want Raí to be happy forever ┗( T﹏T )┛
He can't possibly be Raizel's brother because the traitors said that Rai didn't have enough power (life force) since his fight with his brother and Muzaka. So, it must be someone related to him or perhaps your are right. But we won't know until we have more information regarding the matter. Besides, who likes to reveal the final boss in the climax? No one.
Going by the current plotline it wouldn't surprise me if the last boss was related to Rai, for some reason everything that's currently happening now seems to have roots from the past where the nobles or werewolves betrayed their race. Took them awhile but the consequences are finally appearing.
What if the 1st elder was Raizel's brother, there's way too much use of those blood stones in the Union. Wasn't there a mention before where Raizel's brother was the previous noblesse and he was executed because he used the blood stones? What if he's still alive?! O_O