Is WordPress appropriate for this project?

Interesting project!
I would say definitely WP for the maintenance reasons you mention. Really your project sounds more like a vault or safety deposit box system for people to use, where content would not be shared publicly or even with the other members of this enterprise aside from the creators/families. In that case, everyone enrolled having a separate login to create content at the "Editor" level sounds most appropriate. For this reason, I think you should look into a multisite (to create a network with a shared database) or ManageWP setup (setting up lots of different installations with separate databases). In these instances, each member has a login to his or her separate blog bank, and a super admin would have centralized control of all updates and the general setup, customize dashboard options, etc., all plugins (ie password protect plugin for privacy) at once rather than needing to do each account separately. You could use Advanced Custom Posts to customize the input for sharing memories. You could use Paid Memberships Pro to take payments on the site itself if needed. Just be sure you have enough server space to accommodate your clientele. :-)

If you don't go the multisite route, I'm wondering if there's a membership plugin out there which may suit your needs - where content is shown only to people at different membership levels. Maybe there's a way Paid Memberships Pro or Restrict Content Pro could help here. Using Advanced Custom Posts would allow you to create a "form" with all the different upload and input options for people to add things, and perhaps you could add password protect as a default option on Advanced Custom posts -- though that might get complicated for your users in terms of remembering to enter a PW and remembering the right one to access different content.

hope that helps! .