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.

The diagram of the new (imho way better) Version is quite a bit more complex, but you should get the point.

How do you interact with the UI? Well, every flashlight has a button. This is your input. And you have the LEDs which you can use as an output/feedback. Yeah, it’s wild.

Ungodly ammounts of ultra high power flashlights with open source, customizable UI

First time I saw this, I needed a hot minute to understand what it’s supposed to mean.