I don't really read minds, but my first impression for this one, was that your cousin may have someone else whispering in her ear, that they would rather not see your brother attend. If she'd have expressed concerns with sobriety and the intention of keeping him clean, and asked your opinion on whether or not a wedding would be a great atmosphere, I think that would be a totally different column for me. You are close to the situation and have every right to be upset that she didn't take your replies as a means of conversation. You'd be understanding if it made since, but to you, her idealism in this respect seem like a foreign language.
This is coming from someone who lost their brother in January to a drug overdose, and also saw him uninvited to SEVERAL family events, including holidays, just prior to his death. I stand by those decisions, because it had become a 3 ring circus anywhere he was found, and it ruined the memories we were trying to create as a family. At the time, I was just learning to put my foot down, after years of trying to make excuses, or exceptions, to allow my brother(s) access to the memory lane I was accustomed to. As a family unit, however, we'd made these decisions together, and had decided that we didn't want what could very well be our last memories of him, or our last holidays with him, to be tainted by his behavior, which had become something of a monstrosity and completely out of his character previously.
I also see you are being rational about this being her event, and that you get it, she wants it to be about her, and her choices, and maybe the money is a big factor here as well. Maybe she really doesn't trust that he'll stay sober, and honestly, can you blame her? It sounds like your situation is a very RARE situation where the abuser really is getting the help they need, or wants the help they need, or a combination of both, but that is a dice roll, and some people defend themselves from that, no matter what. Plenty of addicts find a way to hide relapse from their family and friends, ect. The stories and stigma do follow, as you said, so it's not really unheard of for other family members, especially "distant family" to not want to take that risk on what should be a very memorable day in their lives. That doesn't excuse the conversational pull she seemed to have, or lack there of.
I agree that the stigma on addiction is a hurtful one, when it applies to ANY scenario, but there is some historical truth to it. I don't think that, in your case, your brother deserves the scrutiny, as he seems like he has been proving himself for the better part of a year, and sometime there before since he was incarcerated, or completed their treatment program (typically they have to do some combo of both), so maybe she just wasn't ready to hear it? Maybe my initial thought is correct and she is hearing it from someone else?
Bottom line here, comes with your question. "Please tell me that I am making the right choice?"
I wish I could say for certain that you are, but in these situations, go with your gut. Go with your instinct. If your brothers fighting for his life over there, and he is, and he is doing his part to get well, which seems to be the case, then I would absolutely have said, sorry cousin. Your marriage will probably last 10 years (on average) and I need my brother to live for the next 50, so I am going to sit this one out. Sorry if there are any hurt feelings, but I think he needs our support. The fact that the majority of your family (close) is leaning toward the same decision tells me that they all see something in your brother that isn't normally there, and they are willing to give him this chance.
With that big family of yours, do you think this is going to be the last wedding you get the chance to attend? Probably not. ;)
Good luck to you and your family. I know there is no greater struggle than the battle with addiction, and you don't even have to be the addict to experience it!