My uncle has diabetes and is in denial, what should I do?

You can't make him face facts, and you can't make him change. You don't have to enable him (give him regular pop, for instance), but you can't make his decisions for him. The sooner you accept this, the easier life will be for you. You can only be in control of your own children, when they are minors. My mother tries to make me change, and I'm 53, and I quit listening to her a long time ago.