chipping sorry to read how frustrating things are for you. Many of my extended family are vegetarian, plus one close family member has a nut allergy and another is allergic to milk proteins. Trying to create a family meal that everyone can eat is a massive challenge.
When I'm maintaining, or between boot camps, I solve part of my diet with a form of carb-cycling. So for 6 days I eat lchf, and one day I eat more carb (puy lentils, beans, chick peas etc) and on those days I massively cut back on fat. That means I can deal with the bean chilli, vegi burgers or veg and chick Pea curry that my family cook up. They know I won't eat pasta, bread etc. I find I can eat either fat, or carbs, but not both. So while my normal breakfast might be avocado or egg, if I'm going to my dsis for dinner I'll probably fast at breakfast time and have salad for lunch (without the mayo, olive oil etc).
I think that for this to be a long term woe, we each have to find the compromise and balance that makes it possible.