I've had that problem. Not so much with family but very much with friends. All I can say is that I now have the most wonderfully toned middle finger you've ever seen.
My attitude is that I'm not losing the weight for them, so I'm not going to put it on for them either. They can gripe, whine, grumble, complain, or even lay down and throw a full out tantrum. It's not my problem.
I know, it sounds rather harsh, but it doesn't take long for them to realize that I'm not budging and there's no point in discussing it. If they keep pushing, they don't get to be around me in food situations anymore.