I think it's highly unlikely that Akai and Sera's brother is Bourbon... Bourbon was already in the organization at the same time Akai was infiltrating it as an FBI, so it doesn't make sense that he entered the black organization after Akai faked his death -- he would be a newbie if he did, so to speak, and I highly doubt that the black organization will give the job of finding Sherry to a relatively new member. I don't think Bourbon faked his grudge against Akai, either; he could certainly capture Akai to gain the organization's trust, but why would there be the need to make up a "fake" grudge? Akai is the "silver bullet" that could take down the whole organization -- isn't that enough as a motive to get closer to and consequently kill him? Bourbon's grudge against Akai most likely has to do with an incident in the past concerning a deceased member named Scotch.
Just a note: please don't double/triple post -- I've reported it.