EllenP, I am so sorry you are in this situation. I had a mother-in-law who was a binge drinker, and she ruined many holidays (she was Scots, so perhaps you know how that is, coming from the UK yourself). But I loved her until the day she died, even though she and I were total opposites in every respect--in fact I think we admired each others' differences, and that's what gave us this mutual respect.
I don't quite know why you feel guilty. You seem very, very compassionate and forgiving. Please recognize that part of yourself. There is nothing to feel guilty about. We all know the three "Cs": We didn't cause it, we can't cure it, and we can't control it. She has made her choices in life, and you have made what seem to be wise boundaries with regard to your children.
It's tragic, to be sure. But it is what it is. I know what it is to grieve over someone who wasn't the easiest person to actually live with. Please allow yourself that. But know that you have nothing, absolutely nothing, to feel guilty about.
ETA: I do know the experience of feeling guilt with respect to my mother, who was alcoholic at one time, but who spent the last twenty years of her life sober. My AH found her threatening and really cut me off from much contact with her. My deep regret is that I didn't push back and tell him to F off. But I know in my heart that she forgives me, and so I have to forgive myself.