I haven’t read the other responses but I wanted to tell you, first, I don’t see that you are in any situation. Your parents are talking about what they have decided, as of now, and your brother has the responsibility. The prayer is: God’s will be done.
Second, I am in my early 60s, my daughter is 40 and I have a DNR in my purse. I’ve made it very clear I am taking the first exit. My daughter has made it very clear she would want for herself every possible life-saving procedure. We are each other’s medical proxies because we respect one another’s wishes. These issues are about control. As we age, we become less able and the horror is we will end up suffering and have no way to stop it.
Your parents may have chosen your brother because he is the one who understands it is their choice to make, not a church’s choice. It’s not a moral dilemma, it’s obeying the last wishes of the people who are living in the bodies.
As for the suicide threat: everyone says this. Or almost everyone: if they get cancer they’ll commit suicide. My mother said it. But they almost never do. Because it’s not an event, it’s a process, because spouses don’t pass simultaneously, because people have hope. Because they can live long enough to see the last episode of their favorite TV show.
Accept what they are saying and let it go. Unless one of them is very ill right now, in the 15-20 years that is likely to elapse before any of this matters, all kinds of things can change.
In one sense you are right: The parents have free will, and this decision is theirs for “praise or blame, merit or reproach”, as the Catechism puts it, and only God will judge who is worthy of what. The OP does not have the moral authority or the moral necessity to physically prevent other people from violating moral law, even his or her parents. Yes, this does sound like a situation where the OP can only pray that God will intervene so that the parents will not act as they propose, nor be tempted to put their proposals into practice by being put in that situation. “Lead us not into temptation, and deliver us from evil”, yes.
In another sense, however, you are creating a false impression: That is, that this is never a moral dilemma. A Catholic may never feel free to reject the Church’s teaching that suicide is always an unacceptable choice, even if the culpability of the person who chooses it may differ from one to the next. If that is the impression you meant to give, you are entirely wrong. You are also mistaken in the impression that the duty to respect the free will of others removes the absolute prohibition against cooperating with a person who desires to commit suicide. It does not. A Catholic ought to put the wishes of the person they represent before their own opinions, yes, but they do not morally have the option of violating moral law in an attempt to honor the wishes outlined to them before the person they represent became incapacitated. The moral person must explain this before accepting such authority. IOW, respect for the free will of the other means that we don’t lie in order to keep others from making bad choices.
The Catechism explains the nuances of how these two things work together:
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Suicide
**2280 **Everyone is responsible for his life before God who has given it to him.
It is God who remains the sovereign Master of life.
We are obliged to accept life gratefully and preserve it for his honor and the salvation of our souls.
We are stewards, not owners, of the life God has entrusted to us.
It is not ours to dispose of.
**2281 **Suicide contradicts the natural inclination of the human being to preserve and perpetuate his life.
It is gravely contrary to the just love of self.
It likewise offends love of neighbor because it unjustly breaks the ties of solidarity with family, nation, and other human societies to which we continue to have obligations.
Suicide is contrary to love for the living God.
**2282 **If suicide is committed with the intention of setting an example, especially to the young, it also takes on the gravity of scandal.
Voluntary co-operation in suicide is contrary to the moral law.
Grave psychological disturbances, anguish, or grave fear of hardship, suffering, or torture can diminish the responsibility of the one committing suicide.
**2283 *
We should not despair of the eternal salvation of persons who have taken their own lives. By ways known to him alone, God can provide the opportunity for salutary repentance. the Church prays for persons who have taken their own lives.
If the OP were asked by his or her parents to cooperate with their wish to be treated in direct opposition with Church teaching, the OP would have to explain that this would be impossible, and give the parents the choice of choosing to consent to medical decisions that are in keeping with Church teaching or else, sadly, to have to go against the advocacy of the OP for a moral choice and choose a different agent to do what they are determined to do. The OP could not lie and say that he or she would take the course of action that the parents propose while secretly planning to do just the opposite. That lie would be a violation against the parents.
Having said that…there is nothing inherently immoral about making a decision that a particular patient is a bad candidate for some resuscitation effort or other. The finer points of that are a bit much to get into, but suffice it to say that DNRs are not “wrong” and that I’m in no position to judge the paper in your purse. Sometimes, such directives are entirely moral, and there is often a lot of latitude to choose without straying into immoral choices.