EPhantom, you have a lot on your plate. I have seen all your research threads. I can see you are urgently looking for understanding of the problem, and solutions. Unfortunately solutions never come overnight, it always takes time.
I would strongly suggest trying to focus your concern on to yourself, and the things you are able to influence. Your concerns for others in your family sound valid and they do you credit. But if family members are resisting your efforts to help them, that's their decision, right or wrong. So focus on helping yourself. That will put you in a better position to provide help to other family members in the future, when they come round to it. You can never force someone to accept help, and it can be a great source of anxiety to keep trying.
Particularly if there are diagnoses of schizophrenia in your family, I would not recommend pushing those individuals. Seriously, just look after yourself now, and help others only when they are ready to be helped. Your brother sounds like he is ready to be helped.
As for the small children, if they develop T1D, which I hope they won't, the signs will be pretty much indisputable.