Both my parents and I are doctors in India, and my sister and I have been in a situation similar to yours (if not the exact same situation) with regard to not being told our parents' actual wishes in case of some unforeseen health crisis. Moreover we have seen our patients struggle with this problem when one or both parents are suddenly struck down with illness or injury. Indian medical law naturally allows close family members to make health care decisions on behalf of critically ill patients without needing special authorization. The unlikely event of both parents becoming simultaneously incapacitated is a rare but very possible event. And age has not much to do with it. 

A man in his fifties whom we had treated for a minor illness a few years ago died recently of a sudden heart ailment and his wife passed away within 2 days of a brain hemorrhage. Their two young daughters aged 24 and 20 were abruptly in the position of needing to take vital health decisions on their parents' behalf and had been given no instructions about their parents' wishes, although that didn't affect the outcome of those tragic cases.

The problem, of course, is our parents' extreme discomfort with discussing mortality with their children, which is entirely understandable in context, as pointed out by at least two earlier answers. It is the toughest and most unpleasant thing for families to discuss and most parents **including mine** prefer not to go into such a discussion. So I have often been anxious about what decisions I and my 2-years-younger sister might have to take in such a situation. Not to shy away from unpleasant possibilities, this is what I could understand by reviewing the problem:

1. 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. 



2. As it is supposed to be a joint decision to be taken by 3 siblings, you will need to consider your brothers' points of view ***if*** a future decision needs to be taken, and you would thus have only 1/3 responsibility for such decision-making. 


3. By giving multiple children **joint responsibility** to make future decisions (assuming 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 to make their wishes more explicit, but we can't force them to do it, especially if neither of them is currently disgnosed with a life-threatening illness.

Since your father is currently unwilling to discuss this delicate matter, it might be counterproductive to bring up the topic with him repeatedly for the present. I would advise you to try and bring up the matter with your mother instead. She might be more communicative and might also illuminate you about what your father is thinking. You might also discuss the matter with your 2 brothers independently or together to learn what they feel about taking such sensitive decisions in future.


I think your father will become more willing to discuss the particulars of this topic with you in future, after he comes to terms with the implications of your parents' difficult but correct decision to entrust you 3 siblings  the future responsibility of their health care, in the case where they are both unable to decide for themselves. Since your parents are both relatively young and healthy 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 specifically to you based on future health events.