Earlier quoted context omitted.
What is the relevant definition of client and server if they are both peer devices under roughly equal level of control of a single entity?
The client in your family members devices has encryption keys that you do not have in your client or on the server
So what is different in saying „All family devices should see images taken by any device“? They are all clients, including the storage&processing device.
Insisting that E2EE have „ends“ on clients is not useful. It’s much better to define „end“ based on control.