Even if the protein thing was correct, most protein sources arnt 100% protein anyway. Chicken has fats in it and broccoli has fibre (amoung other things).
But seriously this argument thing impacting your home life is ridiculous and very childish. If your uni age cousin isn’t mature enough to stop these petty squabbles than its up to you Duke. I would just stop talking to her about anything fitness and nutrition related if she is the type of person to use that against you.
The thing is, you will get an argument against her from these forums and than you will try to use that against her. She feels compelled to ‘win’ as she is older and is studying so she will than make shit up to your aunty. So even if you win the battle, you lose the war.
Don’t lower yourself to her level because it obviously isn’t helping you any.