He buys minor alcohol

I know it’s none of my business and I stay out of it, but I don’t like that my bf (30) supplies hard alcohol and beer to his 16 year old sister when she asks. He doesn’t like it either but doesn’t know what to do. I told him that he just says no to her, but then he’s worried that she’ll find other connections to get it from people he doesn’t know. What should I do? Break up? Do I just turn a blind eye to all this or pipe up? He knows that I don’t support it. Idk, to me it’s wrong and I’m not a prude, it just bugs me that he’s not being a “good influence” to his only and little sister who he claims to be super protective over. I should also note that their father is a severe alcoholic in which he hates and is ashamed…but at the same time he’s basically to me “starting her while she’s young”