old babe, i'm generally not a confrontational kind of guy and will always first try a work-around rather than a direct hit approach to a sensitive matter, but when push comes to shove, i'm not about to skirt an issue. (did i mix enough metaphors yet?)
if your father has not already prepared instructions, then you need to sit down with family when your father is lucid and your brother is sober and your sister is in town. before confronting dad i would discuss with my siblings dad's options and explore what possibilities exist in his care among the three of you. because brother might think he is capable of more than he might be, you need to keep him in touch with that reality (and that i would approach in a way that makes your brother understand that you simply don't want to burden him in a way that will futher his self destructive tendencies).
i would not gang up on dad during the family meeting. if dad tends towards paranoia, i would not let dad know you had a previous meeting. i would explore with dad just what it is that he expects concerning his future care. then i would find a match or a best compromise between what your dad wants and what you three are able to provide him.
i would make a plan and write it down and have all parties sign to it.
in doing this you should explore issues of aging & dying well. you should get your father to make as many decisions now that he is conscious and lucid so that even if you don't have all the answers later, you will have enough answers to guide you with minimal guilt.
some families fall apart when it comes to end of life issues. if you all keep in mind not what is best for yourself but what is best for each other, you will arrive at a good solution for all.