This is a very real problem, and I understand it well... I have my fair share of relatives who are technologically incompetent.

The solution is to integrate sideloading into the parental controls. There are already existing permissions in iOS to restrict the installation and deletion of apps, so adding a sideloading permission should be straightforward. (They can still leave it disabled by default and bury it a bit behind a few menus and dialogs...) If a family member is really so technologically inept they can't be trusted with their own phone, then you should already be making use of parental controls in some fashion. Set a pin for them which you know and they don't know. It's as simple as that.

Perhaps that's a bit harsh, but we should not be sacrificing these freedoms at all, let alone at a time when there are already existing solutions for protecting those who are vulnerable.

(The relative simplicity of this solution is yet another piece of evidence this issue is not really about the security of users.)