In the event death or incapacity

There are some existing feature requests that may be of interest:

Also, an existing staff response that is relevant:

Based on this, it seems like it would be necessary for each user to set up something of their own to address how they wish their account to be handled. It might be useful to have a specific help page set up with some of the very helpful guidelines @carrieseltzer set out above to give users an idea of how they could approach this and what some considerations are.