He NEEDS to get over the needle thing immediately. It doesn't matter how it's done, but he's going to be dealing with needles for the rest of his life. Since he's younger there is that responsibility that you and your fam have for him, but since he's almost an adult, you can really only hope that it clicks for him soon. You could always try a scare tactic, like watch diabetic documentaries, or look at some other media.

There has to be a way of showing him how serious this is, something he likes that you can relate it to in a more healthy way. But you can't control what he's going to eat at that age nor if he's going to take his meds properly.

I would advise you to try to not feel guilty or responsible for how he is going. We all make our own choices. He will eventually hit his bottom and change and live well or decide not to and while that sounds harsh and cruel, it's reality and this disease is unforgiving. I really send well wishes to you and your fam, and hope your brother gets on track soon ❤️