For example, research shows you can eat lots of nuts and not gain weight. You can starve yourself and only eat processed calories and gain weight. For example, this article from Harvard:
https://www.health.harvard.edu/blog/theres-no-sugar-coating-it-all-calories-are-not-created-equal-2016110410602
Calorie in/calorie out is easy for everybody to conceptualize so it's hard to dislodge that idea, but the best/recent research we have doesn't support it. Not that calorie watching isn't important at all, just that we are learning it matters overall more what kind of calories you consume. No matter how many calories you eat, whole fruit/veggies/nuts/beans/legumes won't make you fat.
As for your family members, good for them! You can certainly drink soda, eat cookies, or whatever and still lose weight if your overall nutrition/exercise plan is good. So a lot depends on the other things you're doing. It's certainly harder, though, with drinking diet soda because we do know that it makes you hungrier, so it's more of an uphill battle.