Jaklackus, my cousin and his DW made a major lifestyle change about 1.5 years ago. He's well over six feet tall, but weighed nearly 300 lbs. and his DW had developed diabetes, although had made significant changes in her lifestyle about two years earlier and things were going well for her.
The went to a doctor at a wellness/lifestyle center and he recommended the following:
At least a 10 minute walk before every meal (including breakfast).
Eat only whole-grain breads (or as close as possible), maximum one slice per meal.
Eat a salad before every meal except breakfast (they were welcome to do that, too, but the doctor warned them it would get old fast!).
Drink at least 12 ounces of water before each meal.
Control portions using the palm of the individual's hand as a guide.
Eat five to six times daily (two meals with salad), planning snacks as well as meals.
Keep in mind, he had specific health issues (he's in his early 60s). He is now down to about 220 lbs and can breath much better. They walk together most mornings for about 30 minutes, drinking a glass of water before and after, and have yoghurt (spelling?), whole grain toast and fruit for breakfast. Snacks are fruit & veggies that they prepare in advance and take to work. My cousin has some difficulty with lunches, as he is on the road most of the time, but he has learned to adapt restaurant food to his needs. His DW usually carries her lunch. Both walk 10 to 20 minutes before eating, as well as drinking the water. Dinner is a glass of water and salad, followed by a small portion of meat (based on hand size for each, so about 4 oz. for him, 3 oz. for her), two veggies and whole grain bread. If they have dessert, it's pretty much what ever they want, but always small portions.
Realistically, your DH and stepson aren't going to change and eat your lighter meals. The best thing you can do is begin a family walking program (or other exercise), have them drink a glass of water and eat a salad before dinner, control portions, and try to get two veggies on their plates each evening! Yes, they can still have mashed potatoes, etc., but in smaller portions!
Also, minimize soft drinks (including diet) and alcohol. It's amazing what a difference just those two adjustments can make!
Once they see changes in themselves and have more energy, they'll be more likely to follow your lead and occasionally eat "light" meals!