They are grown ups, they are not used to having to inform anyone about their health. Or having to inform others about their private information.
They also probably are just used to taking care of themselves and do not want to stress you out.
They also realize you have your own family and probably hear a lot about how busy you are keeping up with your own life and do not want to be a burden to you.
Do not take this personally, this is not all about you. They will inform you when THEY are ready as they always have.
This relationship can go to a different level if you can communicate in a calm way with them. Just calmly tell them you are willing to help them in any way, but you will be honest if you cannot assist them for whatever reason, we never know what may come up.
It will not mean you do not care, or do not want to help. Same for you. If you tell them you need assistance with your family, your health etc..., you want them, need them to be honest, you will not be angry or hold it against them.
And then keep an open honest heart and follow through. Things happen that we never expect. Situations can arise and even though we think nothing can keep us away or impede our willingness to help our patents, life can throw us the strangest curves.
We do our best and then no regrets, because we love each other, we can be honest and not burden our loved ones.