Just tell her in straight terms where her sister's life is going to lead, and provide examples.
Practice these words.
"She's going to end up as a used, wrinkled, discarded old condom, and I feel bad about it because she's your sister and it's going to hurt you to see her end up like that."
Reframe your disgust for her sister as a concern for your GF's wellbeing and she'll be entirely receptive rather than combative.
Practice these words.
"She's going to end up as a used, wrinkled, discarded old condom, and I feel bad about it because she's your sister and it's going to hurt you to see her end up like that."
Reframe your disgust for her sister as a concern for your GF's wellbeing and she'll be entirely receptive rather than combative.
The public will judge a man by what he lifts, but those close to him will judge him by what he carries.