What do you think?
They are always okay because the elder daughter just does not have a choice, they have to be good all the time. If their parents are sick, they need to be there to organize everything and keep this show going. There is no time to not be okay, what a silly question.
Elder daughters just think about everyone else so much that they hardly ever get sick. We think it is because they do not allow themselves to fall ill. They always have some sort of backup strength lying around somewhere that keeps them ticking. In that sense, they are basically indestructible people.