Who is the person nearby if something happens to your parents at 3am?

Not a sibling in another city. The person who can physically be at the house within the hour.

Most of us either have somebody like that or have been meaning to sort it out for a while. If you have, how did you ask them, and what did you agree they would actually do? If money needs to be spent at midnight, how does that work without waking you?

We have written a guide on arranging this, but it is general. What we would rather have on this board is what people actually did, including the arrangements that turned out not to work.