Yes it would be very sad to feel no-one cared if they lived or died, and maybe it could be the last push to REAL seeking after REAL recovery.
How many times have you already shown him, how much you cared for him?
How many times have you already "helped" him when he begged for help.
He had almost lost everything in his life, but still wasn't desperate enough to keep off the drink.
He now has lost his sisters help, and you can see from your own past experience that up to this point nothing has proved to be so terrible and unendurable, for him to stay away for good.
This may be his time to find out that the pain of drinking is WORSE than the pain of living without drinking, and may be his moment of crisis and truth.
Hard as it is, he needs to ride alone, without training wheels and you hanging on.
Having to step back and let him wobble, even fall and skin his knees, is so very painful for we who have done it.
I still get goose bumps thinking about my no longer being there as a "soft landing" for my ABF, and it was even harder than letting my kids on their "big" bicycles with no trainers, for their first solo ride.
What I am totally convinced of is that, (1) had I stayed in contact I may easily have done a "last chance" help run, (for the umpteenth time).
(2) Had I helped him, he most likely would NOT still be sober and still in recovery, and on past performance, he'd have hit the skids anywhere from the next day, week or maybe 6 months at best.
I may hate this accursed disease with a passion, and consider it my enemy, but I cannot fight anyone else's personal battle against it, for them, only my own.
Continue caring about him, just not doing it for him, and do whatever is best for your self and your daughter.
God bless