This has happened in my family. Have a couple cousins (from both paternal and maternal sides of family); niece who WAS overweight but changed her life by becoming a vegan, exercising incessantly and never wavered or backslid. Mother was overweight and I have a son who is overweight also. More prevalent on my Mother's side of the family. All the rest of the family is tall and thin. It sticks out like a sore thumb when people are raised in the same household and have the same activity level and same diet as everyone else, but end up overweight.
Allot of obesity is caused from poverty. They have to survive on starchy cheap foods, boxed dinners, hot dogs & hamburger, junk food from McDonald's, etc. The problem in this group is extreme poverty/ economic issues.
If this country was really the "greatest country on earth", we wouldn't have an obesity problem. Our citizens would have jobs making livable wages, enabling them to afford healthier diets. And we wouldn't have junk food restaurants on every corner. We wouldn't have grocery store shelves loaded with foods that are loaded with high fructose corn syrup, SALT and SUGAR, and now God only effing knows how many GMOs.
One thing that really gets under my skin are those who look down their noses at those with weight issues. Just remember, "There but for the Grace of God go you." Don't blame the people, blame the corrupt practices of our food industry and a government which allows such junk to end up on our tables. DON'T BLAME THE VICTIMS.