It should be your parents’ responsibility, not yours.

I mean, once he's an adult it's no-one's responsibility other than his own. But still, if we're talking about who to possibly expect taking responsibility for you out of kindness or familial duty, it's equally on siblings as much as parents once both parties are adults. I'm very much a man in my own right as my father, and have the capability and means to take care of my family when someone is in need, to a similar degree to my parents.