@Morgen Hm, you're falling into the pitfall of blaming weight on external factors, while weight fluctuations are defined by calories in vs calories out. I don't know exactly how much these external factors might matter, but AFAIK unless it's some chronic disease that messes with your ability to expend the energy stored in your body it's not much at all.

@Morgen May I suggest that the overweight side of your family simply eats too much food, even if it is healthy. For instance, peanuts and other nuts are healthy, but eat a ton of them (and I mean way less than you might imagine) and you'll be gaining weight very quickly; they're really calorie dense.

@Morgen The idea of a diet would be a life-long change, diet meaning what you eat. The problem with simply being "on a diet" is the idea that it's gonna end. If you were to lose weight and go back to eating 12 boxes of Peeps per sitting you'd gain it right back. I don't imagine you care, since you're excusing it all in predetermined factors, but if you ever want to lose weight look up communities like Reddit's loseit or Fitness which will explain it better.

Pro tip: You can lose weight unhealthily by eating a single box of Peeps or healthily, by eating several boxes of brocoli or equivalents, which are very filling and satisfying healthy food which happen to be low-calorie and thus fill you with only a fraction of the calories.