It's so hard to watch our loved ones throw their lives away. My dad did the same thing. He basically just sent the message that he didn't care about his own life and, as his children, my sister and I took it as 'he doesn't care about us then, either.'
Just remember that everyone has their own path and, in the end, we aren't responsible for them. You can set healthy boundaries, tell them you love them, and then just let God (or your Higher Power of choice) take care of them. We can sit with them, we can care for them, but in the end we are the ones who sit and watch them fall down the deep hole of addiction and it hurts. Going to Al Anon meetings truly did help save my sanity and I do hope you are finding support IRL.