I don't think it makes sense to spend so much thought on a hypothetical situation. If the brother blows up, the brother blows up. It's not a pleasant situation but it shouldn't have any impact on your boyfriend's sobriety.
I'd be more concerned that your boyfriend is setting himself up for a relapse by acting like his brother is some kind of drunk tornado that could land on him at any time. He always has a choice in the matter. If he thinks going to his brother's place will endanger his sobriety, then he shouldn't go. That's not going to be the most fun conversation in the world, but it's also pretty cut and dry in my opinion.