"Tell them that nobody ever wants to be an alcoholic; that alcoholism flowers in some people but not in others; and that Grandma just drew the wrong straw. And then add this reassuring fact: It won't happen to them, because you're going to teach them how to avoid the problem, no matter what straw they draw."
My only addition to her advice would be to find an Al-Anon group or other support group for family members of alcoholics for on-going support. Read the entire column here.