Perhaps you can compensate for eating less by praising the food really enthusiastically? I mean, they're bound to notice your fabulous weight loss, so perhaps talk about how lovely the food is, and what a treat it is, and then it won't seem like you're rejecting the food when you don't eat so much. You could also suggest that you're not used to big meals anymore? Less of you to keep up and all.
Best of luck! Remember, it's only a week, so even if you have to eat everything your family hand you, it can only be a few pounds - frustrating, but not worth beating yourself up over.