Hi, Aithyne.
It must feel frustating seeing your father in pain and drinking again and not be able to do anything to change the situation. And then on top of it, you're feeling triggered by old stuff from your past at the same time.
It sounds like you and your sister have done all you can do by expressing to him your concerns. The rest is up to him. Had he been in a program of recovery during his years of abstinence? Is he familiar with AA?
Take care of yourself.
I know that decision may feel difficult or even selfish if you've spent your life worrying and trying to fix other people, but it's the only action that will help and it's the only action that will work. Keep working on your own sobriety, job #1.
Are there any Alanon meetings in your area that you could attend? It may help you to share your concerns. And you'd receive much needed support in return. Is it from Alanon I heard the quote, 'you didn't cause, you can't cure it, and you can't control it'?
Hang in there!