I have to preface this by saying I'm not trying to be insensitive, but I don't understand and think some perspective from the other side might help. I've had a DNR (and full advance directive) in place since my 40's (in full health). It was my own personal decision, and I honestly don't think it's anybody else's business. The decision is mine to make. As it is theirs.

Not that any reason is needed, but among the reasons for mine is that I watched my grandmother die on life support. She wouldn't have wanted it that way (but did not have papers attesting to), and I don't want it that way for myself. I'd be pissed AF if someone thought it was their business to oppose what I want for myself in such an incredibly personal issue.

Truly, I don't mean to be insensitive, but it's their decision. It's about them. It's not about you.

Alternately, you could take the tack that it *is* about you -- in the form of a kindness from them -- that they have taken care of this piece of the puzzle before it becomes an issue so as not to have to potentially throw unimaginably difficult decisions at the rest of the family. Better that people declare what they do and do not want NOW. It's when people refuse to deal with it that things can go REALLY bad.

Food for thought.