You could use Syncthing for local folder syncing between devices. It’s been really reliable for me.

The siblings could run
tailscale ping nodeIP/nodeName
This not only shows if they are connected via a DERP (relay) but also retries to connect them directly a few times.
If that doesn’t work out it can sadly be quite hard figuring out why a DERP is used instead of a direct connection.

There is a project that can run some backgrounds, but is very experimental and IMHO not suited for running constantly as a desktop background. WPE is way too deep in specific windows features to be able to run on linux smoothly.