I am in a similar position to you, my sister is an alcoholic and my Mum keeps bailing her out and picking up after where she has fucked up. I do think my parents avoided discipling her as a child becasue she was so tricky and would kick off all the time. Her alcohol problems began aged 13, and it was not nipped in the bud enough, but we know more now than we did before (ie. I would have her to counselling from age 13, she actually didn't go to rehab until she was 31 for the first time).
Its a tough one because she has periods where she is ok. All I get from my Mum is she is mentally ill and needs help. Thing is she avoids all "proper" help and at the moment she likes getting so pissed she cant function for weeks at at a time. One day being sober we have #healing #onit 'backtomyself posts on social media. Then she goes to ground again.
Her behavious over the years has destroyed many relationships, my parents have split up after 44 years of marriage as they couldn't agree on how to manage it (she is 43 now!) and my relationship is strained with my mum dad and sister. We are divided as a family My brother had given up. My mum is currently paying for an expensive court proceedure to try get access to her 9 year old son, he doesnt want anything to do with her either. Its very sad, and she never ever puts her hands up and says sorry. She just buries her head.
I dont blame my mum at all, she is her daughter and she wants her to be well. But my parents are entering their 70s where they should be relaxed and enjoying their retirement and its the opposite. My mum is so stressed she doesnt sleep, has had several falls recently and I am sure its all stress.
Anyway, just wanted to say you are not alone, the guilt of doubting her is hard to deal with, let alone the worry she will be found at the bottom of her stairs. She is a beautiful and smart woman, she has wasted her life and could have achieved so much. Sadly I can't see her pulling herself out of it anytime soon and the lies keep coming. She has recently had a provisional diagnosis of borderline personality disorder. I don't doubt that she has "demons" and she needs help, but at the moment she does not want it. What she cannot seem to link is her drinking has literally caused every single issue she ever had (breakdown in relationships/ friendships, losing jobs, not reachng her potential, putting herself in dangerous situations, injuries, losing her driving licence twice, getting a reputation locally). I feel for you and your family, its very hard indeed. She is so loved too, she has had more support than anyone else I know.