Rosie,
There are mostly big people in my family, too. And you know what I've observed about that? It seems to be due to shared bad habits, rather than shared DNA!
Family gatherings centered around the kitchen, and featured lots of fried chicken, heavily sauced and cheesed casseroles, breads, desserts... sit around and eat until you can't eat anymore. Rest awhile and repeat. No wonder most of my aunts and great aunts and cousins were so big!
I have two aunts who are quite heavy. Both of these ladies eat like truck drivers, yet have insisted for decades that their weight problems are due to genetics and metabolism, not bad eating habits and gluttony.
One of them even tried a SlimFast diet once. But you know what she did? (This is the truth...) She just substituted SlimFast for her beverages. I actually watched her eat a Big Mac and Fries along with her SlimFast.

Because of her "slow metabolism", SlimFast just wouldn't work for her. She's always had serious denial issues. EVERYTHING bad that happens in her life is someone else's fault, never her own.
I guess what I'm trying to say is: don't let "it runs in the family" blind you to the true reasons for the weight struggles.
Think about what you said here: "I soon find that if I start enjoying life with some decent food even for a few days or a couple of weeks, I began to bulge." It sounds like your family might be like mine- we've been trained to think that enjoying life is closely tied to feasting on not-so-healthy "comfort food."
One of the first hurdles that I had to overcome was learning to separate "enjoying life" from *any* food issues. Now, eating healthy (well, MORE healthily, anyway) is second nature. I don't need food to feed me emotionally.
Next time you're with your heavy family members, try to really notice how they eat, and see if your family might be like mine. You might need to teach them how to steam a few veggies!
