

Ideally? I’d say something with support for 2 drives, mirrored.
Without a price to define “not crazy expensive”, I’d say take a look at QNAP and Synology 2-bay devices, pick what fits the price range. I’d put it on the network and mount it on the PCs you want to as a drive to drop things in.
Then I’d add one more thing - 3-2-1 for anything critical.
- Three copies of all critical data should be made on a regular basis. Original data + at least two backups
- Two different storage types should be used to store the data to minimize the chance of failure.
- One offsite location. Could be an encrypted copy of a backup on cloud storage, could be a drive at another home.
How critical data is would be up to you and your parents, I just want to note that a single backup at your home is not going to be helpful if there is flooding or a fire or whatever that would damage both the originals and the backup device.
So location is by instance and not by user?
That seems an odd (and kind of problematic) design…