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.

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I just looked into this and I will never see chainsaws in the way i’ve seen them before. As far as I understand it it was revolutionary at the time and saved lives, but it was absolutely gruesome either way.
For anyone who is as innocent as I was 10 minutes ago and wants to change that

Chopping wood?

Stretch it!