Sounds like it is time to show the folks you mean business. I know this isn't easy when family is involved but being direct is best. Tell them you do not want the cake/ice cream (unless you're opting for a small piece - make sure that is YOUR choice, not just caving in to them). If they continue, I would consider walking away. Go to another room, or leave altogether. This may sound drastic but it sounds like they need to know this isn't just a lc issue, it's about respect for your choices.