It is rare for both parents to be seriously ill and incapable of taking medical decisions at the same time. It is generally much more likely in our clinical experience that one parent falls critically ill and health care decisions are taken by the other parent in consultation with their adult offspring.
As it is supposed to be a joint decision to be taken by siblings if both parents are rendered incapable, each sibling will need to consider the others' points of view if a future decision needs to be taken, and each would thus have only 1/2 or 1/3 responsibility for such decision-making.
By giving multiple children joint responsibility to make future decisions (assuming in your case that you do not have any higher responsibility as the eldest child: or else please edit question to clarify) parents are actually reducing the decision-making burden of any one family member.
It would certainly be more useful for parents to make their wishes explicit regarding future medical decisions, but we can't make them do it, especially if neither of them is currentlyhas yet been diagnosed with a life-threatening illness.
Since your parents are both relatively young and healthy now, it is to be seen as a currently hypothetical long-term contingency planning for the future and you can expect both parents to communicate their wishes much more explicitly to you in due course, based on more specific future health events.