I agree that it's none of your business and in no way is it appropriate to tell your grandmother anything that you found out doing your internet searches.
I do think that if you wish to reestablish contact with your uncle and cousin, then you can do that. You can email your uncle about your disorder "for his knowledge" and try to friend your cousin on FB (I'd even send her a PM letting her know about the hereditary disorder "for her knowledge".) If they bite, they bite. If they don't, drop it.
If they do decide to reopen contact with you, I don't think there's anything wrong with saying "grandma's been really worried." But it's really up to them to decide what, if anything, to do with that.