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.