NOT BUYING MY GF WEED

My girlfriend f16 and I m17 have been going out for like 7 months and for the past 3 months everytime her dispo runs out she asked me to buy her one. I have unfortunately, I shouldn't have I don't like the idea of a fuckin 16 year old smoking weed, I want her to quit. It's not her fault her fuckin sister let her smoke since she was 13, SIDE NOTE: what kind of sister would do that!!!!. That's so fucked up but anyways, I keep confronting her, telling her I don't want her to smoke anymore but she keeps telling me she just want to have "fun" that she's gonna be and adult soon and she gonna have so many responsibilities and stuff like that. I know I'm not in the wrong for telling her to quit but I would really like to to hear some reassurance.

I just got off an argument with her about it and she basically said that she'll quit. But she sounds upset and mad at me. Ik im doing the right thing but this hurts.