When you eat a large amount of food you avert an attack of anxiety. You actually begin to eat long before the anxiety is fully developed. As soon as you get an indication that anxiety is on its way you begin to use food as your psychic defence. Eating large amounts of food, especially white carbohydrates, sugar and fat, can be effective in reducing anxiety in the same way as some drugs are. It is just as difficult for a person with an eating disorder to do without sugar and fat as it is for a drug addict to do without drugs.
It seems that you have a problem in relation to parties and especially concerning your parents in-law. Perhaps you don't find it very enjoyable to eat dinner with them every Sunday, but for some reason you won't even admit it to yourself.
Next time you get a craving for sandwiches, try not to eat any, but lie on a bed, close your eyes and allow your feelings to arise. Don't try to explain away feelings with reasons, just let them come and overwhelm you. Accept the tears and the anger. When you by makeing contact with your real feelings it will be more difficult for you to do things you don't want to do. You are perhaps not especially fond of parties and want to devote your time to other things? It may be so that you would feel better if you didn't have to meet your parents in-law so often or even not at all.
If you become more aware of what you feel, then you can surely solve the problem in a better way than by eating sandwiches. As long as you eat, you will never find out and therefore can't tackle your problems in a realistic way.