First of all, congratulations to your boyfriend on his sobriety. It's a shame about his brother, but as you probably know, there's not much you can do to change an alcoholic until they decide they're ready.
I think your boyfriend is wise to avoid being around his brother when he's drinking. It's not just that the temptation would be there...... it's all the drama, too. If your BF hasn't already, he might want to check AlAnon out......