This stuff is not hard and fast. My parents forbid alcohol in the strongest terms, though they were not teetotalers. I’ll admit I drank underage but I was for the most part a very, very responsible drinker until I stopped completely in 1994.
I know parents who took that same approach, and their kids were major binge drinkers. That includes my brother, who nearly killed himself driving drunk. Yet I know families that handled it with supervised drinking and did well (minor league social drinkers) or did poorly...I have a high school friend who goes through wine like Bacchus.
The bottom line is it’s very unlikely that your niece’s parents caused her alcoholism, any more than my parents caused both binge drinking and teetotalling in their boys with the exact same approach to alcohol. Mainly I think it was the fact that my brothers and i ran with very different crowds.
Her problem is all George Bush’s fault. :-)
My husband's reaction to an alcoholic father and 5 alcoholic uncles that kept the entire family in turmoil was to never take a drink for fear he would end up sleeping in vomit like they did.
No one in my family ever drank very much because it gave us all terrible headaches.