I can understand why you would want to avoid MacOS and Windows, but I wouldn’t overlook Chromebooks. They’re inexpensive and the setup procedure is simple. Also, having a larger screen and a physical keyboard could be beneficial for some users.
Try downloading Chrome OS Flex (the successor to Cloud Ready) onto a USB drive and booting a computer from it (be careful not to install it as it will wipe the drive).
Overview -
https://chromeenterprise.google/os/chromeosflex/
Direct link for the free home edition -
https://support.google.com/chromeosflex/answer/11552529
If you’ve got an old computer that you no longer use, you could repurpose that and see how your stepgrandmother gets on with it. In my experience, Chrome OS Flex is not as good a solution as having a dedicated Chromebook - the last one I tried wouldn’t play video on the BBC website - but it’s a good way of testing it out.
There's also Chrome Remote Desktop for troubleshooting -
remotedesktop.google.com
This video might be useful -
If you do go down the Apple route, I’d start with an iPad first as I think the bigger device will be easier. You can decide about an iPhone later.
Also, look to see if there are any courses at local libraries that may help her.