As far as her family goes, I would recommend just being really firm in expressing what you do and don’t want to do and make sure you make it crystal clear that your declining to eat has nothing to do with food and instead has everything to do with your personal choices. Meaning, don’t focus on defending the choice itself, but rather focus on defending your right to make your own choices in general. If they’re the kind of people who ask more than twice, you may want to be a bit sterner like “my answer isn’t going to change” etc. You’ll also have to make sure you show that they can earn your appreciation in other non food ways in case that’s part of the issue.
You should probably try to correct your girlfriend’s view of food though, since it sounds unhealthy and it sounds like it will continue to be a barrier in your relationship.
When I first started dating my girlfriend she baked at least once a week. Right now she’s cooking a high protein meal prep for lunch this week because we’re both cutting fat together this month. It was a huge process to get her to this point, and I would be a fool if I thought I was ever really “in control” of it or if it was all my doing. Off of the top of my head I would say I used the following strategies to various degrees of effectiveness:
Tell her straight up that it’s not okay for her to pressure you into eating. Find something she doesn’t like being pressured about and make the comparison. Never ever budging on this stance is crucial, because it will take some repetition for her to properly wrap her head around this view that you have, and it will take longer for her to accept it.
Fight fire with fire. Respond to her saying you’re rejecting her as a person by saying that she is too: you’re the kind of person who doesn’t like being asked or told to eat things and she’s rejecting that.
The key here is to convince her that you know something about eating healthy that she doesn’t know without trying to be a smart ass about it. You’re trying to help her develop a fuller and higher understanding of and relationship with food that will supersede her current relationship with it. If that’s not a good option for her particularly then maybe try to get her to come to her own realization that her relationship with food is either uneducated or one-dimensional.
Convince her by any reasonable means necessary that calories and macros are important. This will depend highly on the individual. For example my mom kept “trying” to lose weight for decades and I convinced her to start tracking her calories by going with the “this stuff has been known for hundreds of years, there are no breakthrough diets” approach, which played against her appetite for a prescribed fix by playing to her senses of generational pride and elitist pride by knowing an “older” method of accomplishing something that all these youngins couldn’t achieve with their fancy fad diets that people kept trying to sell them.
Ensure she doesn’t have any structural misunderstandings. For my girlfriend, she was always focused on “portion size.” She would say things like “chipotle is unhealthy because the burrito has so many calories.” I had to convince her that that was nonsense because she could easily choose to only eat half the burrito and throw out the rest.
Continue redirecting her hobby. For example you could find some good YouTube channels with healthy meals or whatever and watch them when she can overhear.
When you cook, always know the macros and always tell her. In the beginning this will be about desensitization, later you can discuss more what your goals for the day are and how that meal you made fits into it.
If you have friends who are physically fit and eat healthy, expose her to them and their eating habits whenever possible. This will obviously be less effective if your girlfriend is one of those magically thin high-metabolism girls.
Be super appreciative when she does cook something healthy.
Yeah re-reading this i kind of rambled and I’m not sure whether any of what I wrote may or may not be directly translatable to your situation but I’ll probably leave this here for a bit in case it was helpful to you somehow. Cheers mate.