I was out walking around thinking about your situation, and came back and read the last several post about your BIL entering treatment, leaving treatment, etc.
I was thinking about all the people who suggested that you try Al-Anon, and I was thinking that Al-Anon might not be the right place for you to get help, but now I think that I was wrong.
This isn't over. Your BIL's recovery is fragile (a lot of people relapse). Your SIL is likely an alcoholic, (more likely than not in my opinion). The kids are still at risk. Your extended family is a little more awake than they were, but still in denial.
Al-Anon is mostly for people who are affected by those who drink. Maybe you haven't been directly affected by the drinking, but man, are you surrounded by it! I think that in the coming days, weeks, months and years as this drama plays itself out, the support and insight you could get from participating in an Al-Anon program would be very useful.
People who are attending their first Al-anon meeting are told to attend at least 6 meetings. This is partly because each meeting is different, but I think it's also because it takes some time to figure out why a group of people sitting around sharing stories and not giving advice is going to be useful to you. But it does work, and it does help. It's worth 6 hours of your life to check it out...and even if you don't think you need it now, it's a great resource, maybe for some of those other members of your family who maybe do need what the program offers.