Can you work on food quality first? Homemade nuggets with gluten-free "breading", grain-free "granola" bars, grain-free pancakes, a different nut butter (instead of peanut), good quality yogurt with honey for sweetener (slowly reduce the sweetener) etc. It will be more effort on your part,
Does your fiance know that your family fared better on that way of eating? If they know they felt better, could they make the choice to eat Primal if you had Primal and non-Primal offerings on the table (keeping the pasta separate from the sauce, with spaghetti squash for non-grain eaters, for example)?