Honey, the only thing you can do is let it go. You're not just up against habits here, but pride. People like to think that they're "strong enough" to handle anything, that their moral fiber will somehow compensate for anything they do to themselves. If you say, "I'm worried about your health," they'll hear, "I don't think you're smart enough to take care of yourself, and you don't deserve to enjoy life." The reason they turn eating healthily into a joke is because they're on the defensive--I don't like it when my mother tells me I should exercise more; I can imagine how much I would resent it if the power balance were the other way around!
So I'm sorry, but all you can do now is to focus on your own pursuit of health and ignore them. Trying to make yourself into an example will just goad them on. But if you don't flaunt it, and just go about your business, you might have an effect later down the line. At first, I had a lot of difficulty refraining from force-feeding my sweetie broccoli, but you know what? After three years of just giving him what he wants and making separate meals for myself, he's started to request brown rice instead of white and V8 for breakfast with no prompting at all!
One day, if they feel like it, they might change. And if they don't, it's no reflection on you.
