OP, if I'm being honest here, I think your first post is a bit of a misleading one especially considering your subsequent ones.
In your first post you presented your situation as a dilemma you were facing. However after you posted in reply to someone who said something very reasonable:
"You don't get it, do you? Never mind."
I came to the conclusion that actually this isn't about helping you make a decision as deep down in your heart you've made the decision and you do seem to have confirmed that when you've now said:
So - no children for me. 
This changes your OP for me.
AIBU to seriously consider not having a child for this reason ?
You are not unreasonable for deciding not to have children for ANY reason. You don't have to justify it to anyone else. It seems clear that you feel like you don't feel like you could cope with certain situations. However that smacks to me that reflects something about your life NOW rather than any future you have. You sound like you are struggling to cope and are in need of support in dealing with your brother's issues. And that is being transferred to your feelings about having children. The thing is any child you have is not your brother or indeed any of your other relatives. They are an individual in their own right who might deal with problems in the same way, but equally they might cope in completely different ways and have support from other sources. You just don't know. What it comes back to is, how you see the illness and how you are coping with it now and not being able to see any alternative way of life living with it.
I think your comment Of course there aren't any guarantees with any child which perhaps means I shouldn't be a mother at all, as bluntly I want to be a parent, not a carer. perhaps says a lot.
And as others have pointed out there is no guarantee that any child you have will have it. So this reflects how you view risk and how you are viewing this as a probably that your child WILL have it, rather than the likelihood that they won't. I think in some ways you are doing yourself a massive disservice here. You are so focussed on the fact that your child won't be able to cope because of their genetics you are missing that their mother must be a hell of a woman with some bloody great genes to deal with living in a family where several people have lived with it and is capable of looking after her brother.
But I suspect this is a bit of a mute point as I think you have decided not to have children anyway and the purpose of this thread isn't to help you make that decision but to validate that decision and help you come to terms with it and maybe hear from others in a similar position. I think you are struggling, and for that reason despite your protests I think you might benefit from counselling, maybe indeed genetic counselling, to come to terms with that decision.
I think having a child is the biggest decisions you will ever make. I think to take it so seriously is one that many don't do and full credit to you for doing so.
But when you say:
Perhaps it's easy to say you wouldn't regret it in the abstract but the reality of phone calls at 3 in the morning, of crying, of terrible, crippling anxiety, of financial problems, of seeing someone you love completely disengage from life - would you still not regret it? And would you regret it if you know you would die and leave them helpless and alone in the world?
This could be ANY of us. Regardless of what condition your child may or may not have. And again its reflecting what's going on in your life right now, not telling you what's going to happen in the future if you have children. You could have a child and it could be a life changing gateway to a more positive future. Unfortunately unless you open that door way you'll have to deal with the 'what if' demons as well as the 'if only' demons.
Like I say, I really think this is about your current situation and coming to terms with a decision you've made in your heart rather than asking if you are unreasonable.
I hope you find the peace you are looking for.