You say that the common depictions of scissoring is not representative of tribbing as a whole. Which I completely agree with. Because tribbing is a word for “rubbing the vulva against something for stimulation”.
But that doesn’t mean that scissoring isn’t real. It’s a specific form of tribbing. And there obviously are people who have done and enjoyed that, and are now confused why people tell everyone that it isn’t real.
Sure, it may not look like the common porn depiction. But it may surprise you that the porn industry isn’t really good at representing actual sex in general. And just because you yourself don’t enjoy this position doesn’t mean others no not as well.

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

I either buy mp3 or download from youtube.

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.