My Girlfriend wants me to have sex with her sister

A couple months ago me and my girlfriend decided that we want a child and that I would be the one getting pregnant.But she wants the baby to be a part of her and me which can't happen.So we dropped the whole thing.But recently she was talking about having a baby again and she said that I could just have "SEX" with her twin sister.(Her sisters transgender).I told her I wouldn't feel comfortable with that.So she suggested just putting the sperm inside of me.I just dont feel comfortable with having her siblings sperm in me so now she's mad at me cuz of that.šŸ˜ž