Don't enable encryption for the room until everyone is there, otherwise if someone sends a message, late arrivers will not be able to see those.
But do enable encryption when everyone has joined.

Also, set up automatic key backup (with a good, long random password stored in a password manager, maybe even yours if they don't want one) for all of them. It's in the settings. If you don't do so, and someone in your family loses access to all of their login sessions, they won't be able to decrypt past messages, because they don't have the keys for them, and no one client will send them.

Both of these things stem from the same problem, and hopefully it will be fixed some day.

I'm trying to do the same thing. I don't know if the room settings need much tweeking. But the biggest obsticle is having family members download, install, create an account, join a space/room, and change app/account settings, without my physical presence to help them with the process.

Wait until element x releases, please. My family had terrible experiences with the element app

Would highly suggest you to stay away from Matrix and Element for now when it comes to using it with non techies. It‘s simply not there yet. It‘s slow, tedious to use, buggy and the ux is just bad. It‘s also not necessarily more private than the popular choices if used improperly which is imo very likely it you have your aunt use it lol.

Is there any feature of element that you need or why did you pick it? You could ideally just use Signal for group chat and group (video-) calls. Or even whatsapp.