Now he's going to IOP MONDAY? Great--three more days of nonstop drinking. I doubt he will go on Monday. OK, I've officially changed my opinion about his present commitment to recovery. Someone could have a 2-3 day slip, fall not too far and get busy redoubling their efforts, but the further they slide, the further they slide. This stalling does not look very promising for the immediate future. Not to say he won't get back to it, but it could take days, weeks, or months.
As for his sister, you might suggest SHE check out Al-Anon.
Sorry things are taking this turn. Once an alcoholic picks up a drink, it's a complete crapshoot where it will wind up. I was hoping (as I know you were) that it was just a slip, but it's looking like he's determined to shoot himself in the foot. This is the problem with people who have all that INTELLIGENCE going for them--they have egos and pride to match, and it keeps them from getting humble enough to go back through the doors.
Keep taking good care of YOURSELF--no matter what he does.