Hi
@Lindad, this is not your fault, in the same way that your own diagnosis is not your fault and neither is it either of your son's faults. I think it's natural to have these feelings, particularly as we still don't understand enough about what brings on t1 diabetes.
I am t1 and the only one in my family with diabetes (t1 or t2), and was diagnosed a little over a year ago at the age of 29. Of course, diagnosis is difficult, but I am so so grateful that I developed it now rather than when I was child or teenager (every cloud ☺).
I know it's hard, but try not to be so hard on yourself. I think you know in yourself that there's nothing you could have done to prevent this (and vice versa, you can't bring it on either). Be strong and continue being a fantastic mum. Your youngest son is lucky to have 2 family members who understand and can give advice, and of course to provide the support he needs. All the best x x