I find that what JayEll says works really well, if combined with the right kind of attitude. And I don't mean your attitude towards food, but the face you show the world. I have found that if I act as if I couldn't care less about food - as if I was thin and never been fat - people don't usually pester me. If you come across as confident and sure of what you want, then the less pushy at least will back off. Maybe your family is different though.
Personally when I mention that I want to keep my calories low and I don't want this or that for some reason people see that as a way in to start pestering and questioning. But if I just smile a bit and say "thanks, I'm good" they just leave me alone.

Never did understand though why people feel the need to tell you what to put in your mouth. Food really is a socially loaded subject!