I moved out for eight months and it has definitely helped my diet
But I understand the situation - I'm almost astonished by my family's choices.
here's the advice that was given to me (now live at home, and follow it with great results)
Buy your own groceries and adopt your own space (cupboard, fridge, vegetable crisper). 25$ a week will get you fresh vegetables, lean ground beef, fruit juice, milk, eggs, etc. Having a stock of healthy choices is key.
Eat/cook your own meals - it's not hard, complicated, expensive, time consuming at all and is very rewarding.
Snacks - Chop up carrots, celery, brocolli, spinach or whatever. Throw it in a container filled with water in the fridge. Seriously, every time I look in the fridge, I grab a handful and go to town.
As for the social pressures, I have little advice. I avoid my mother as much as possible, but I assume that's not an option for most people

I've "already eaten" whenever my family is heading out to McDonald's