Similarly, one of your nephews has a friend with parents that don’t lock their liquor cabinet, which means despite all the laws not allowing sales of alcohol to minors, they still have access to it.

I think what your sisters are doing is fine—they’re sending a signal to their kids that this stuff isn’t “good” and though they’ll undoubtedly encounter it in the world, they’re now going to be inherently biased a certain way. And that’s kinda the best you can hope for.