Sometimes taking it "one day at a time" is still too daunting. Sometimes it's "ten minutes at a time".
When my sister was in jail the second time (I think the second time?) her bond had gone up enough that my mom couldn't foot the bill, so the calls came pouring in. I was standing outside my doctors office when I got one call, and I took it. I had finally gotten an appointment with this doctor, he is one of the best fertility doctors in the region, and I was standing outside crying on the phone while my appointment slipped by. I was SO MAD at myself for allowing that to happen. Nothing was even accomplished on the phone call - I didn't bail her out, she didn't care at all about anything that I was going through. Luckily he let me reschedule after he saw my file that I had brought in from a previous doctor out of state.
I'm not trying currently, but I have health problems that make me a very difficult case, and I've had two surgeries already while I get ready. I'm 36 now so the clock watching is going on over here also. Anyway this is all just to say that I totally get it, and I also get how much even a phone call can completely derail the best laid boundaries and plans. You sound like you've got better boundaries defined than I did at that time, I had no idea what was going on and no support, all I was doing was trying to mimic what I had seen the interventionist do on the TV show. Yeah, wow, I have come a long way!
You need to put yourself and your desire to bear a child ahead of your brother. Not just a little bit ahead, but for right now, WAY ahead. Because the stress, like you said, is a major factor. When he decides to get clean (or if) he will need to focus entirely on himself for at least a year, so your role is really minimal in a healthy recovery, anyway. It's OKAY to not always be right there, waiting to hear what comes next. The best thing you can do is focus on keeping yourself calm, happy, and stress-free, and strengthening those boundaries - they are going to be an absolute necessity once there's a little one involved.
This was a novel. Sorry about that. It sounds like we really have a LOT in common. Feel free to PM me if you ever need to just vent or chat or whatever, I work on the computer so I'm around a LOT during the week.