New and unsure where to start

I'm an older child of an alcoholic. My sister and i have known about his drinking problem since i was a teenager. About 10 years ago it went to another level, and it keeps getting worse and worse.

My mum and dad divorced when i was 5 (i'm 31 now) so we only saw him on a weekend, and we now know that back then he was what i'll call a 'functioning alcoholic' - working and seeing us on a weekend, while still drinking. Then 10 years ago he had some sort of break, and it has just gotten worse and worse. To the point that this summer he lost his job, after being off on the sick for 7 months. And now he doesnt have work, he is just drinking constantly. My sister has recently backed off after my dad let her down this year by not paying for her wedding like he promised (leaving her to try and find money for final payments 5 months before the actual wedding). I get why she is struggling to re-connect with him at the moment, and i cant blame her.

But we used to share the 'burden' - now i feel like its on my shoulders and mine alone. I have never been super close to my dad, but the idea of him sat in his flat drinking himself to death weighs very heavily. I call him because if i dont, nobody else will. But when i do call him, hear the mess he is in, it leaves me in a mess. I know i cant help him if he wont help himself. But i live in constant fear of him dying, to the point where i have panic attacks after i get off the phone to him. 

There is plenty more detail to the story, maybe i'll write it down at some point, might help.