Honestly I think telling her to fuck off is about the only thing you can do for most of it, though I guess on the serving thing you could try telling her that if she cooks something really healthy when you come over you could eat more of it? Like obviously if you're eating healthy and the meal is pasta you would take a small serving, but if it's something like chicken with rice and broccoli on the side you could have one normal serving and then fill a whole plate with broccoli alone and still be good as far as your diet goes.

I mean obviously you might not want to eat a whole plate of broccoli whether because you might not like it or just the principle of it, but that's the only sort of compromise that make things easier on you that I can think of.

Though that said if you have picked up an exercise hobby that you're really into you could take the I'M SUPER INVESTED IN MY NEW SPORT route and blame all your diet habits on it being best to excel in your sport, whether or not you're legit that into it.