You will drive yourself crazy if you keep thinking about whether he will get sick and the siblings run your life.
The best thing you can do is to consult a solicitor. It costs money, but you need to know where you stand legally, and only a solicitor can tell you. You could also ask about legal aid, because from the sound of it you have absolutely no money of your own except what you can salvage from the housekeeping, unless you get yourself a part time job, and there are those around, and if it is far enough away, he won't even know.
It is up to you. You cannot lose by consulting a solicitor, and you certainly will lose your self respect if you keep going round in circles worrying about it.
It is wrong for a man to marry a woman who he does not think capable of managing the household if he cannot at some time in the future, unless he knows that some time in the future he will not be able to. You might get hints of that if you trace his family tree and see what his ancestors died of. It would put your mind at rest on that point if they did not have dementia or other disabling diseases. He most certainly should not have had children if there are.
If you want to put your mind at a little bit of rest, so that you can be sure of any sicknesses that can be passed down the generations, you can join one of the many family tree sites. I traced my family tree back quite a way, and my late husband's back to 1590. You do not have to go back that far. You can find their death certificates in your search and see what they died of.
If nothing else, it will keep your mind occupied on something positive, and those sites have experienced helpers who love to help.