This is such a tough one, Puzzled. I have a similar situation in that my parents, now just my mother, enables my alcoholic sib. Always has and, I guess, always will. I think part of my parent's enabling, like your father's, is rooted in guilt. He was not planned, and she didn't particularly want a third child. Also, she took some medication during her pregnancy (prescribed) that she believes damaged him in some way. He always struggled with school.
I too struggle with patience and compassion for them both, as they live together and both have dementia. Hers is from old age, his is from too much alcohol.
I have vented about them here, and stated that it is very difficult at times to see them together.
A wise person on this site replied to one of my venting postings in this way: "yes, it's hard to watch, but she has been doing this all her life. She isn't going to change. And at the end of the day, it is her life and she should be able to live it the way she wants. Even if you don't agree and you don't like it."
That post has stayed with me. I try to step back when my sib is being a d**k, which is most of the time. I tell myself that it's not my circus and not my monkeys.
Al-Anon has helped me with some of the resentment and anger that I feel toward both of them sometimes. And sometimes, when my sib is stomping around like an idiot, which upsets my mother, I will tell him in no uncertain terms to knock it the heck off!
But mostly, not my circus, etc. etc.
Sorry this has become such a long post. Bottom line, our parents are going to do what they are going to do. All we can do is try to learn from their experience and try not to repeat it. Peace.