Well, there IS some truth to 'marry a girl you marry her family' (or his). If they are serious, you need to know this sooner rather than later. Doesn't mean you'll actually DO anything, but a family's attitude toward things like money, kids, religion, etc are important to know when considering marriage.
That said, I'd put myself on an ice flow before letting my son's life get messed up because MY poor planning. As for illness, we have both a will & a 'living will' that describes what we'd want in various instances.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh